How to read a scientific paper
How to read a scientific paper: The QDAFI method of structured relevant gist
New York University
Most academic fields document and communicate the knowledge that was gained by
doing research in the form of research papers. Consequently, these papers are full of
the latest information as to where the field is right now and where it is likely to go
next. They also constitute a historical record of how the empirical edifice of any
given field came to be. Thus, being able to gainfully read original research papers is
an indispensable skill. Unless one is able to do that, one will have to rely on a
summary by someone else, and this person might well have a limited understanding,
misaligned interests or even both.
So being able to access this information yourself is important. But there are
roadblocks. Lots of them.
Research papers are written by experts, for experts. Thus, research papers tend to
be full of specialized jargon while also leaving many important things unsaid as the
authors can rely on the shared assumptions and tacit knowledge of the intended
audience. However, there are times when non-experts want or need to read a
research paper, as in the case of students, journalists or simply interested laypeople.
This is a problem, as the lack of common ground will make much of what is in the
paper seem like gibberish (Clark & Brennan, 1991). Even worse, one cannot
remember what one cannot understand (Bartlett, 1932), which means that even
after heroically struggling through a paper, very little of this hard-won information
will be retained long term.
Moreover, a key intention of authors is to convey to other researchers what exactly
took place in the study – in principle one should be able to replicate it from the
information in the research report itself, something that is an increasingly critical
consideration (OSC, 2015). Consequently, research papers tend to be chock full of
technical details, not all of which are equally important to the overall point of a
paper. There is probably no correlation between how easy something is to
understand in a paper and how important it is. People tend to remember what they
are able to understand – as non-experts won’t necessarily know what is important
and what is not, this poses another problem: They might remember particulars that
are easy to understand, like that there were 57 participants in the 2nd experiment,
but which are of negligible significance in the bigger scheme of things. This issue is
exacerbated by the fact that the retention of minutiae likely comes at the expense of
more relevant information. People do not retain an accurate and comprehensive
memory of the information that was actually presented. Rather, people’s long term
memory performs a kind of compression operation and this compression happens
in a semantic space – whatever meaning someone is able to extract is retained.
Known as memory for “gist”, it is often surprisingly sparse and a caricature of the
original information (Reyna & Brainerd, 1995).
These issues illustrate that reading a research paper poses a formidable challenge to
non-experts like students or journalists. The first step in meeting and overcoming
this challenge is to acknowledge its tremendous magnitude. Unless someone is a
scientist in the same field as the author, one should not expect to be able to read a
research paper as effortlessly as something that is written for a general audience –
like a newspaper or magazine. One should not expect to be able to get anything
useful out of reading research papers willy-nilly. The good news is that being able to
read a research paper is a skill and skills can be acquired. For this purpose, we
developed the “QDAFI” method. The QDAFI method allows to gainfully read
research papers in general. It was designed to exploit the structure inherent in
research papers as well as to leverage the cognitive aspects of reading research
papers, some of which we outlined above. For instance, if we know that people have
a tendency to retain only the gist of some passage, we can anticipate this and get
ahead of it by trying to make sure that the gist that is being retained is *relevant*
gist. There are other such considerations as well. Research in cognitive psychology
suggests that the format of information – shaping how it is encoded – is of critical
importance for retrieval (Craig & Tulving, 1975; Roediger & Karpicke, 2006). As we
will usually want to retrieve information from the paper in form of a targeted
question such as “what did the study find”, it makes sense to encode the information
in the form of answers to the handful of most likely subsequent questions .
Auspiciously, every research paper – regardless of field or content – is more or less
structured in a way that is conducive to this approach. Put differently, the QDAFI
amounts to a mining operation – it will require a bit of work, but we’re essentially
trying to extract nuggets of relevant information from the paper by digging for them
in places where we know that the authors put them, buried within the products of
an academic filibuster that attempted to make the authors look smart and please the
reviewers. Critically, the QDAFI is – deliberately – already all gist, and gist of the
kind that we’ll need if we are to discuss the paper in the future, in other words
structured and relevant gist. It consists of the answers to five questions and these
answers have to be brief – between 1 (ideally) and 3 (maximally) sentences each,
fitting on a single page (without fiddling with the margins or fonts). This constraint
necessarily enforces the discipline to focus on the most relevant information – the
gist. Which is all that anyone will remember anyway. But it is better to anticipate
these things up front. Putting in the necessary work to condense one’s
understanding of a paper to a few sentences probably does wonders for one’s grasp
of the paper all by itself.
Specifically, the QDAFI asks for answers to the following 5 questions:
Q: What was the question that the authors tried to answer?
D: What did the authors do to answer the question?
A: What was the authors’ rationale?
F: What did the authors find?
I: What is the interpretation of these findings with regard to the initial question?
Let us now look at each item (and how to answer it) in detail:
Q: What was the question that the authors tried to answer (with this paper)?
Every good research paper starts with a question that the paper is trying to answer.
It is worth restating it explicitly – as succinctly as possible – here, as without a
question to frame what happened next, nothing will make any sense. Whatever else
the authors did, they presumably did it to answer a question. State it, here. The first
– “Q” – portion of the QDAFI can usually be addressed by looking at the Introduction
section of papers. If you are lucky, the authors state it explicitly, if not, you’ll have to
read between the lines to cobble it together yourself. One complication is that there
might actually be two questions explicitly or implicitly stated in the paper. The one
question that always needs to be stated here is the “specific research question” – the
study was done to answer which question? This could be anything, e.g. “what is the
influence of changing the font color on memory retention?”. The second question –
and the one that usually pervades the spirit of the introduction is the “theoretical
question”. This question is typically the reason why the study was done in the first
place – it’s not arbitrary, but touches on some larger theoretical issue or controversy
in the field, e.g. “what is the influence of context on memory? So the Q portion
should ideally be one sentence (to state the specific question) or – if you manage to
spot the theoretical question as well, two sentences. Not more. Importantly,
consider (for now, not when doing the QDAFI) all the things that are also typically in
an introduction, but won’t make it into the QDAFI – because they are irrelevant for
that purpose. For instance, introductions often spend a lot of effort on describing
why the question is an important one – this makes it more likely for the paper to be
published in a more prestigious journal. But for your purposes, this is irrelevant.
Presumably, you are interested in the paper and know why it is important (to you).
Also, introductions tend to dwell on all the other things that have already been done
to answer the theoretical question, highlighting why they think more research
needed to be done (i.e. their paper). None of this academic soul-searching and the
paper trying to justify its existence needs to be of your concern. At least not for
purposes of the QDAFI. Realizing this can be liberating, as non-experts in particular
can be intimidated by the wall-to-wall citation blocks of papers common in many
introductions, none of which the non-expert has read.
D: What did the authors do to answer the question (as reported in this paper)?
Here, we want to state – as succinctly as possible – what the authors actually did in
order to answer their research question. This information can be found in the
Method section of the paper. However, this is tricky, as this section tends to be –
along with Results the densest of the entire paper, full of technical details of interest
to experts. It is very easy to get lost in irrelevant minutiae here. How are you
supposed to summarize all of this information in a single sentence, or – at most –
two? It is not easy, but it is quite doable. If the study reports an experiment, the
most natural way to do this is to report what parameters were varied (the
independent variables) in one sentence and what was measured (the dependent
variables) in another one. If the study is observational, you can devote one sentence
to what was measured and one to under which conditions the measurements were
taken. Unless it is absolutely critical to the question and interpretation and if it
makes a difference whether you remember it in a couple of years, it does not belong
into “D”. Be extremely stingy in terms of what you include to report here. If the
paper reports on a multitude of experiments, you can potentially use a third
sentence to outline how the conditions and measurements changed over the
experiment (e.g. “they controlled for potential confounds such as x, y and z”). Make
sure to be as specific yet high level as possible. Also make sure that you do not get
lost in details, yet at the same time don’t leave out anything crucial. Unless it is
crucial to the question that the study asked, details such as the precise numbers of
participants, their gender, etc. do not belong here. Again, like in “Q” recognizing this
can be a great relief.
A: What was the authors’ rationale (in this paper)?
This section is perhaps the trickiest of all, yet it is at the heart of the whole QDAFI
method. Stating the rationale necessitates you to understand what the authors were
trying to do and paraphrase it in your own words (as it often goes unstated), so if
you can state the rationale, you probably have a good grasp of the paper as a whole.
If you can’t, then it is quite possible that you were just copying what you think are
the most relevant parts of Q, D, F and I. So the rationale (the A) is crucial indeed. The
point of the rationale is to get at something like: “What was the idea behind the
study?”, “What is its logic?”, “What allows the authors to infer anything useful about
the question from what they did?” or “Why did the authors do this thing in D and not
something else” (out of the infinite number of things they could have done to
answer their research question). The rationale or idea links the specific research
question with the actual methods they used. Done properly, it sets up what the
authors can conclude from any possible outcome, given their methods and question.
Without a valid rationale, nothing can be concluded from any given result. Contrary
to popular belief, the data do not speak for themselves. Rationales are rarely spelled
out explicitly – as the experts in the intended audience will either grasp them
intuitively or a given rationale might even be part of the shared culture of a field.
Rationales can be tricky to track down, but the information necessary to make an
informed guess is usually distributed in Introduction and Method. For instance, one
could have a theory that more intelligent people have faster neurotransmission. If
this is the case, we would predict that they are quicker to respond to a stimulus.
Now we have a rationale. We can now meaningfully interpret the results, e.g. if more
intelligent people are slower, this falsifies the theory, whereas it would be empirical
evidence in favor of the theory if intelligent people were indeed faster. This is a far
cry from randomly measuring reaction times and correlating it with intelligence. It
is a rationale. The stronger the rationale, the more conclusive the results will be, for
or against the theoretical issue at stake. Depending on how complicated the
rationale of the study was, this section might require anywhere from 1 to 3
sentences, but not more.
F: What did the authors find (in this paper)?
This is perhaps the most straightforward section of the entire QDAFI. What the
authors found – the results of their study – are usually found in the Results section
of the paper, and quite explicitly so. The biggest challenge here is to identify the key
findings that are relevant to the question the authors asked initially and to
understand them, as they are usually expressed in an extremely technical language,
often involving statistical notation no less. Again, the discipline imposed on us by
the QDAFI method comes to the rescue. It forces us to identify the core of the matter.
Which of the many results reported in a paper – the average paper in Psychology
contains no less than 11 p-values (Nuijten et al., 2015) – are actually relevant to the
original question? Some authors like to build up to their main result, others hedge it
with many side-considerations. Typically, the reviewers also requested additional
analyses that were of personal interest to them and which the authors obligingly
included in the paper, so as to get it published. Try to find *the* key finding – that
matters in terms of the original question. This is another reason to do the previous
part – the rationale well. The rationale will tell you which finding – or pattern of
findings to look for in the Results section. Usually, this key finding can be stated in a
single sentence. Sometimes, if the findings are particularly surprising or
complicated, a 2nd sentence will be needed to elaborate. Never are more than 3
sentences appropriate or necessary. Given the plethora of statistics typically
reported by the authors ,this can only be accomplished by focusing on the high-level
big picture – what matters in terms of the question, as spelled out by the rationale.
Finally, focus on the facts. What did they actually find? State this as dispassionately
as possible. Do not mention anything about what they – or you – think these findings
mean. That is part of the next – and last – section.
I: What is the interpretation of these findings with regard to the initial
Put simply, what do the authors think their findings mean? How do they address or
answer the original question? This is usually straightforward – the authors will tell
you what they think, in the Discussion section of the paper. But that’s not all there is
to the “I” part of the QDAFI. This part consists either of 1 sentence or of 3 sentences.
The first sentence is what the authors think the findings mean in terms of the
original question. The 2nd sentence states an issue that threatens this interpretation
and the 3rd sentence then states what you – consequently think these findings mean.
For instance, say some authors wanted to know whether the ability to delay
gratification in childhood is a major predictor of life success (Mischel et al., 1989).
Say they find that it does. They conclude that this shows that the ability to forego
small but immediate rewards for large but delayed ones is the bedrock of individual
success as well as societal functioning. If you agree with this interpretation, you are
done with this one sentence. However, it is possible you might spot a problem. For
instance, you could point out that the authors neglected to measure trust of the
individuals – trust that the later reward will in fact materialize, as the experimenter
promises. It has been shown that children from lower SES backgrounds have –
perhaps reasonably so lower amounts of trust in promised future outcomes (in an
unreliable world, going for the safe, immediate outcome might be the rational
strategy). In addition, low childhood SES is linked to poor life outcomes. In other
words, there is a major unaccounted for potential confound here. While interpreted
by the authors as ability to delay gratification, what might really be going on is that
they measured trust level as a proxy for SES – which is already known to be linked
to adult life outcomes. Thus, if you identify this problem, *your* interpretation of
these findings might well be that unless ruled out in a future study, it is quite
possible that this study shows that low childhood SES is linked to poor adult
outcomes, likely mediated by trust. So far, so good. The key problem with this
section is to identify an issue (you can remember this section as the 3 “I”s:
Interpretation of the authors, Issue, Interpretation by you) that is actually an issue –
in other words one that is so serious that it threatens the interpretation of the
authors. It is understood that every research paper suffers from a large number of
problems. This is necessarily the case, as we live in a non-platonic world. Life is
complicated. Things don’t always work out. That’s ok. However, issues are problems
that are true showstoppers. A real issue is usually something like faulty logic (e.g.
the authors interpret a correlational study causally, when they should have done an
experiment), a potential confound like the one we outlined above or a serious
technical problem (e.g. the method the authors used does not have the sensitivity to
reliably detect the results the authors claim they found). Truly severe stuff. It is not
something trivial like the number of participants, as we always would like to have
more research participants (unless the study is seriously underpowered, Wallisch,
2015), nitpicking that the monitor on which the stimuli were presented was not
large enough (unless it was really too small for the participants to reliably see the
stimuli), or other minor technical issues. All studies have limitations but not
necessarily issues that threaten the conclusions that the authors draw (I) from their
findings (F) with respect to their question (Q). It might not be reasonable or feasible
for non-experts to find such fundamental flaws of a study. Instead of manufacturing
issues, it is fine to stick with the interpretation of the authors – succinctly stated in
one sentence – instead.
If you do that, be sure to focus on the big-ticket item – what does it mean with
respect to the main issue/question at hand. There are often plenty of considerations,
but focus on the biggest one, the one you mentioned in Q.
And that’s it – that’s the QDAFI in all its purposeful simplicity. Done right, this will
easily fit on one page and – hopefully – be memorable for years to come. Less is
definitely more. Ideally, each of the 5 sections of the QDAFI should be tweetable
(140 characters or less), but this is not a strict requirement.
Once you have done a couple of these QDAFIs, you will start to appreciate what a
useful tool they can be. Being able to do a QDAFI is a beneficial skill to have mainly
for three reasons. First, research papers are not written in a fashion that lends them
to be read like any other writing. This is a problem, as they look very much like
other written works, but if you attempt to just read through a research paper (like
you do with any other writing), you probably won’t get much out of it. Second, longterm
memory works in a fashion that is radically semantically reductive over time.
Short-term memory can be deceptive in this way. After just having carefully read a
paper, all the details will be readily available in your mind at that point (assuming
that you understood it). But this won’t last. Most, if not all of the details will be
forgotten over the course of the next couple of days, weeks, months and years. The
only thing that you will remember about a paper in a couple of years is only what
your brain thinks the point of the paper was, just the most important things. This is
a problem if you need to remember the content, but your brain at the time thought
that the most remarkable thing about the paper is that all authors’ last names
started with a “G”. Expertise in any field consists of the building up of a database of
highly semantic knowledge over time. This is – by the way – not a problem of
memory. It is decidedly a feature – the brain needs to make sure that it is not getting
too cluttered with utterly irrelevant information. The QDAFI anticipates this
housecleaning business and makes sure that the most important points about the
paper are also the ones that you focused on when “reading” the paper. You’ll lose the
irrelevant stuff anyway. We need to help your brain to figure out which the
irrelevant things in the paper are up front and the QDAFI is one way of doing this.
That’s why it is critical that it is so short. The shorter the better. Your brain will
make short work of the memory anyway. Third, being able to do a QDAFI goes
beyond the ability just to read papers – although it will provide a very powerful lens
for that as well (as we just discussed). For instance, you can summarize the
literature very efficiently like this. This will come in handy if you should ever be
asked to write a review paper or write an introduction to your own research. If
you’re reading this, chances are that you will be in a position like that at some point.
Moreover, it is the method of choice to structure an abstract if you are ever asked to
present your own research. It maximizes information transmission with the
minimum amount of text in a clear and cogent way. Finally, you can even use it
(anticipating that other people will QDAFI your papers) to plan your own studies
and write research papers about them.
Now, doing a QDAFI is hard. It is really, really hard. That much is understood, so
please don’t be discouraged. The best way to learn how to do them is to do them. It
is well worth it. If it isn’t already clear from the discussion above, rest assured that
anecdotal reports are overwhelming that it is an extremely valuable skill. In classes
where we taught the QDAFI, students email us – sometimes years later – that this
was indeed the most valuable thing they learned in the class. We are currently
trying to figure out how to ethically study the effectiveness of the QDAFI empirically,
which is tricky. As we (think we) know how helpful the QDAFI is, it would be
unethical to teach one group of students how to do this, but not the other group of
students and then test retention of articles at the end of the class. Conversely, if we
required groups of students to do QDAFIs vs. free-form summaries of papers in
alternating weeks, we would have to teach the QDAFI to both groups, and we
couldn’t rule out that the “free-form” group also implicitly does a QDAFI in any given
week, so this comparison would perhaps only be valuable for the very first week,
before the free-form group was taught how to do the QDAFI, but that seems like a lot
of work for little useful data.
But that need not to be of your concern. We believe chances are good that it works –
due to the principles laid out above, we use it ourselves and we also teach it – to
great effect to our students. We hope you will find this method useful as well.
Paper 1: Mueller, P. A., & Oppenheimer, D. M. (2014). The pen is mightier than the
keyboard advantages of longhand over laptop note taking. Psychological Science,
Q: Does taking class notes by hand yield better academic performance than taking
them on a laptop?
D: Participants were asked to either take notes on a laptop or by hand while
watching TED talks. They were then asked to answer both factual and conceptual
questions about the material presented in the talks.
A: It is believed that using laptops to take notes yields poor performance because of
distractions. However, even without distractions, if laptop use leads to more shallow
processing, it could detrimentally impact performance on conceptual questions.
F: Students who typed notes on a computer performed worse on conceptual but not
factual questions compared to those who wrote them by hand.
I: Taking notes on a laptop negatively affects performance in response to conceptual
questions compared to taking notes by hand, which could be due to the fact that
taking notes on a laptop seems to encourage shallow processing, such as copying the
Paper 2: A Substantial and Unexpected Enhancement of Motion Perception in
Autism by Foss-Feig et al. (2013). Journal of Neuroscience, 33, 8243-8249.
Q: Do autistic individuals exhibit enhanced motion perception? Are there reliable
psychophysical markers of autism that could transcend subjective diagnostics?
D: They show moving gratings varying in size and contrast to groups of autistic
children and controls. They measure how long they have to show these stimuli until
they are perceived accurately by the study participants.
A: It is believed that autistic individuals might suffer from a deficit in inhibitory
neurotransmitters. If this is the case, they should show less spatial suppression,
which is believed to rely on inhibition and therefore be quicker to see large stimuli.
F: Autistic individuals are more sensitive to perceive motion at all stimulus sizes, but
they are not better at low contrasts.
I: It is not the case that autistic individuals show less spatial suppression – rather,
the low contrast results suggest that there is abnormal gain control in autism.
The authors neither measure nor report reaction times; as autistic individuals are
known to exhibit longer reaction times, it cannot be ruled out that the results are
simply due to a speed/accuracy tradeoff. This does not mean that duration
thresholds can’t be used as a psychophysical marker of autism, but it might not be
for the reasons the authors think. [For a more detailed discussion of this issue, see
Wallisch, & Bornstein (2013)]
This method was originally inspired by Stephen Kosslyn’s “QuALMRI” technique
(Kosslyn, personal communication).
Bartlett, F. C. (1932). Remembering: An experimental and social study. Cambridge:
Clark, H. H., & Brennan, S. E. (1991). Grounding in communication. Perspectives on
socially shared cognition, 13(1991), 127-149.
Craik, F. I., & Tulving, E. (1975). Depth of processing and the retention of words in
episodic memory. Journal of experimental Psychology: general, 104(3), 268.
Foss-Feig, J. H., Tadin, D., Schauder, K. B., & Cascio, C. J. (2013). A substantial and
unexpected enhancement of motion perception in autism. The Journal of
Neuroscience, 33(19), 8243-8249.
Mischel, W., Shoda, Y., & Rodriguez, M. L. (1989). Delay of gratification in children.
Science, 244(4907), 933-938.
Mueller, P. A., & Oppenheimer, D. M. (2014). The pen is mightier than the keyboard
advantages of longhand over laptop note taking. Psychological Science,
Nuijten, M. B., Hartgerink, C. H., Assen, M. A., Epskamp, S., & Wicherts, J. M. (2015).
The prevalence of statistical reporting errors in psychology (1985–2013). Behavior
research methods, 1-22.
Open Science Collaboration. (2015). Estimating the reproducibility of psychological
science. Science, 349(6251), aac4716.
Reyna, V. F., & Brainerd, C. J. (1995). Fuzzy-trace theory: An interim synthesis.
Learning and individual Differences, 7(1), 1-75.
Roediger, H. L., & Karpicke, J. D. (2006). Test-enhanced learning taking memory tests
improves long-term retention. Psychological science, 17(3), 249-255.
Wallisch, P., & Bornstein, A. M. (2013). Enhanced motion perception as a
psychophysical marker for autism?. The Journal of Neuroscience, 33(37), 14631-
Wallisch, P. (2015). Brighter than the sun: Powerscape visualizations illustrate
power needs in neuroscience and psychology. arXiv preprint arXiv:1512.09368.
research paper for this QDAFI:
In Cold Blood: Characteristics of Criminal Homicides
as a Function of Psychopathy
Michael Woodworth and Stephen Porter
This study investigated the relationship between psychopathy and the characteristics of criminal homicides
committed by a sample of 125 Canadian offenders. It was hypothesized that the homicides
committed by psychopathic offenders would be more likely to be primarily instrumental (i.e., associated
with premeditation, motivated by an external goal, and not preceded by a potent affective reaction) or
“cold-blooded” in nature, whereas homicides committed by nonpsychopaths often would be “crimes of
passion” associated with a high level of impulsivity/reactivity and emotionality. The results confirmed
these predictions; homicides committed by psychopathic offenders were significantly more instrumental
than homicides by nonpsychopaths. Nearly all (93.3%) of the homicides by psychopaths were primarily
instrumental in nature compared with 48.4% of the homicides by nonpsychopaths.
In terms of its impact on the victim, the victim’s family and
friends, and the financial resources devoted to its investigation,
homicide is the most severe form of antisocial behavior. Despite its
extreme negative consequences, homicide is also one of the least
studied and most poorly understood forms of antisocial conduct.
An obvious reason for the lack of research on the psychology of
homicide is that it is uncommon compared with other forms of
antisocial and violent behavior. However, it remains a significant
problem within all cultures and nations (e.g., Daly & Wilson,
1988). Homicide is a heterogeneous phenomenon, associated with
different contexts, motivations, and types of perpetrators. For
example, some homicides are highly calculated, instrumental acts,
whereas others are characterized by an apparent lack of premeditation,
occurring in the context of an emotion-laden dispute or in
response to a situational provocation. Research leading to a more
thorough understanding of the factors associated with different
forms of homicidal violence could have both basic and applied
implications. As an example of the latter, if specific psychological
characteristics in offenders were found to be associated with
characteristics of the crime itself, it could allow investigators to
reduce the large field of suspects in many homicide cases (e.g.,
Woodworth & Porter, 1999).
The present research focuses on one psychological construct
that is highly relevant to the criminal justice system (see Hart &
Hare, 1997). Psychopathy is a personality disorder characterized
by a profound affective deficit accompanied by a lack of respect
for the rights of others and societal rules (e.g., Cleckley, 1976;
Hare, 1996, 1998; Porter, 1996). The current state-of-the-art diagnostic
tool (see Fulero, 1995) in the assessment and identification
of psychopathy is the Psychopathy Checklist—Revised (PCL–R;
Hare, 1991). As operationalized by the PCL–R, psychopaths are
manipulative, callous, remorseless, impulsive, irresponsible individuals
who often engage in diverse antisocial behaviors. With a
prevalence of 15%–25% in the federal offender population, psychopathy
is an important risk factor for recidivism and, more
specifically, for violence (e.g., Grann, Langstroem, Tengstroem, &
Kullgren, 1999; Hemphill, Hare, & Wong, 1998; Kosson, Smith,
& Newman, 1990; Lyon, Hart, & Webster, 2001; Salekin, Rogers,
& Sewell, 1997). For example, Serin and Amos (1995) found that
psychopaths were about five times more likely than nonpsychopaths
to engage in violent recidivism within 5 years of release.
Porter, Birt, and Boer (2001) investigated the complete criminal
career and community release profiles of 317 federal offenders.
They found that psychopathic offenders consistently committed
more violent and nonviolent crimes than their nonpsychopathic
counterparts from late adolescence to their late 40s (also see
Harpur & Hare, 1994). Important from a risk management perspective,
although the release performance of nonpsychopaths
improved with age, it got worse for psychopaths as they got older.
Psychopathy also is associated with more severe forms of sexual
violence (e.g., Brown & Forth, 1997; Hare, Cooke, & Hart, 1999;
Kosson, Kelly, & White, 1997; Serin, Mailloux, & Malcolm,
2001) and targeting multiple victim types (Porter, Campbell,
Woodworth, & Birt, in press; Porter et al., 2000).
Overall, therefore, the dominant clinical conception of a psychopath
is a dangerous person who preys on others across the life
span (e.g., Hare 1998; Simourd & Hodge, 2000). Given this
observation, a relationship between psychopathy and some forms
Michael Woodworth and Stephen Porter, Department of Psychology,
Dalhousie University, Halifax, Nova Scotia, Canada.
This research was graciously supported by an operating grant to Stephen
Porter from the Social Science and Humanities Research Council of
Canada (SSHRC) and a grant to Michael Woodworth from the American
Psychology–Law Society (AP-LS). This study was conducted as part of
Michael Woodworth’s graduate thesis under the supervision of Stephen
We express appreciation to the Correctional Service of Canada for
allowing and helping us to collect the data for this study. In particular, we
thank Doug Boer, Jeff Drugge, and Jeff Earle very much for their invaluable
assistance. Thanks to Jeff Hancock, Mary Ann Campbell, and Angela
Birt for comments on an earlier draft of this article. Thanks also go to
Peyton Harris, Mary Ann Campbell, Naomi Doucette, Kris Peace, and Matt
Lafond for their assistance in coding the data.
Correspondence concerning this article should be addressed to Stephen
Porter, Department of Psychology, Dalhousie University, Halifax, Nova
Scotia B3H 4J1, Canada. E-mail: email@example.com
Journal of Abnormal Psychology Copyright 2002 by the American Psychological Association, Inc.
2002, Vol. 111, No. 3, 436–445 0021-843X/02/$5.00 DOI: 10.1037//0021-843X.111.3.436
of homicide seems likely. For example, based on their pathological
personality traits, and in light of previous research on psychopathic
aggression in general (e.g., Cornell et al., 1996; Serin, 1991), it is
plausible that psychopaths engage in more instrumental, goaldriven
(e.g., to obtain money or drugs) homicidal violence relative
to nonpsychopathic offenders who may engage in predominantly
reactive, spontaneous violence (e.g., in the context of a heated
argument). No research to date has examined this issue.
Although there are various conceptions of violent behavior,
many authors have noted that violence may be best understood by
considering the external goals of the perpetrator. Bandura (1983)
viewed aggression primarily as an instrumental and goal-driven
behavior contingent on external rewards and reinforcement. He
suggested that most aggression is committed with the “pull” of
various resources or gains in mind. In general, instrumental or
“proactive” violence occurs when the injury of an individual is
secondary to the acquisition of some other external goal. For
example, this form of aggression typifies the activities of organized
crime groups who often commit strategic and planned violence
as a means to achieve an otherwise nonviolent goal, such as
money or drugs (e.g., Amir, 1995). Other researchers have argued
that emotional or “internal” factors play an important role in
violent behavior. In Berkowitz’s (1983) conception, aggression
can be conceived as a hostile and angry reaction to a perceived
threat or dangerous situation. Accordingly, the primary goal of
aggression is to defend oneself from a perceived threat or to react
against a perceived environmental frustration. Such reactive aggression
encompasses impulsive, immediate, and emotion-driven
acts in response to a perceived threat, danger, or insult.
More recently, some have observed that this dichotomy, although
theoretically important, may oversimplify a highly complex
behavior with multiple motivations and manifestations. That
is, it has been argued that violence may contain elements of both
instrumentality and emotionality/reactivity (e.g., Bushman &
Anderson, 2001; Cornell et al., 1996; Poulin & Boivin, 2000) in
both children (e.g., Crick & Dodge, 1996; Dodge, 1991; Poulin &
Boivin, 2000; Vitiello & Stoff, 1997) and adults (e.g., Block &
Block, 1992). Block and Block (1992) observed that the
“expressive–instrumental extremes are ‘ideal types’ that seldom
occur in pure form” (p. 65). Further, Kingsbury, Lambert, and
Hendrickse (1997) noted that often there is an overlap between the
two major types of violence. In fact, Bushman and Anderson
(2001) argued that the instrumental/reactive dichotomy is of questionable
validity in categorizing all acts of aggression by incorrectly
assuming that all acts of reactive and/or hostile aggression
are “automatic” whereas all acts of instrumental aggression are
“controlled.” Nonetheless, according to a number of researchers
(e.g., Eaves, Douglas, Webster, Ogloff, & Hart, 2000), a determination
of whether violence is primarily instrumental or reactive
may be one of the most relevant criteria in assessing risk for future
violence and for treatment prognosis in criminal offenders (also
see Heilbrun et al., 1998).
The main purpose of the present study was to examine possible
differences between homicidal violence committed by psychopathic
and nonpsychopathic offenders and to focus primarily on
the instrumental and reactive elements of the crime. According to
Cleckley’s (1976) classic conception, the behavior of the psychopathic
individual often is motivated by a clear external goal rather
than the powerful emotions of rage or despair associated with
crimes of passion and that many psychopaths, in fact, displayed a
profound deficit in emotional reactivity. Given their propensity
toward violence in general (e.g., Hart & Hare, 1997), the use of
instrumental or proactive violence would not be unexpected from
the criminal psychopath. The general lack of empathy or remorse
and the presence of shallow emotions (e.g. Hare, 1991, 1998)
could be manifested in the context of their crimes and, more
specifically, their homicides. On the other hand, psychopathy often
is associated with impulsivity and poor behavioral controls (and
problems with temper control), suggesting that violence by psychopaths
might be highly reactive and inordinate to a particular
situational provocation. A small number of studies have investigated
this issue (Cornell et al., 1996; Hart & Dempster, 1997).
Williamson, Hare, and Wong (1987) examined the nature of the
violent offenses in a group of 101 Canadian offenders. They found
that psychopathic offenders frequently were motivated by material
gain or revenge (45.2% compared with 14.6% of the nonpsychopaths)
and did not appear to have been in a state of heightened
emotional arousal at the time of the violent act. In contrast,
nonpsychopathic offenders appeared to have experienced more
emotional arousal during their crimes: 31.7% of the nonpsychopaths
exhibited strong emotional arousal—such as jealousy, rage,
or a heated argument during their offense—compared with 2.4%
of the psychopaths.
In more recent work, Cornell et al. (1996) examined the relationship
between psychopathy and violence in 106 male offenders
from a medium-security state prison. The authors operationalized
instrumental violence as violence that was goal-driven and required
planning without an antecedent of provocation. Reactive
aggression was defined by an absence of planning or goals and,
instead, involved a dispute or interpersonal conflict with the victim.
They found that, across their criminal histories, psychopaths
(as classified using the PCL–R) were more likely to have committed
instrumental violence than nonpsychopaths (who were
more likely to have committed reactive violence). Instrumental
violence was most commonly associated with a self-reported lack
of arousal or anger during the commission of the offense. Further,
the victim of instrumental violence was typically a stranger,
whereas reactive violence often was associated with high emotional
arousal and a close relationship with the victim. There also
is some evidence for a link between psychopathy and instrumental/
proactive aggression in nonincarcerated samples. For example,
Chase, O’Leary, and Heyman (2001) found a relationship between
psychopathy and the use of instrumental violence by male spousal
assaulters. In their sample of 60 abusive married men, no individuals
who were classified as being reactively aggressive were
psychopathic, compared with 17% of the men who were classified
as instrumentally aggressive.
There is disagreement about the extent to which the
instrumental–reactive distinction is useful in conceptualizing the
violence committed by psychopathic and nonpsychopathic individuals.
Dempster et al. (1996) investigated the institutional files
of 75 adult male violent offenders participating in an inpatient
treatment program. Although psychopaths were found to have
committed more instrumental violence, they also had displayed
impulsive behavior in the context of their offenses. Based on these
findings, Hart and Dempster (1997) concluded that even if psychopathic
individuals commit more instrumental crimes, they may
be “impulsively instrumental.” It is possible, then, that psychoPSYCHOPATHY
AND HOMICIDE 437
paths could engage in homicides that, although goal-directed, are
highly impulsive and involve little planning (having elements of
both instrumentality and reactivity). Thus, in addition to homicides
that appear to be exclusively instrumental or reactive, some primarily
instrumental homicides may contain a reactive component,
and some primarily reactive homicides may contain an instrumental
component. Conceptually, this complex or diverse violent behavior
seems plausible because the current construct of psychopathy
encompasses both affective/interpersonal traits, known as
Factor 1 characteristics on the PCL–R (e.g., glibness and superficial
charm, pathological lying, lack of remorse, and shallow affect)
as well as Factor 2 characteristics associated with a chronically
antisocial and unstable lifestyle (e.g., a need for stimulation, impulsivity,
lack of realistic goals, and promiscuity). There is some
evidence that instrumental aggression is related to the Factor 1
features of psychopathy, whereas reactive aggression is more
associated with the Factor 2 characteristics (e.g., Patrick & Zempolich,
1998). The present study addressed these issues and was
the first to specifically examine the relationship between psychopathy
and homicidal violence.
The sample was composed of incarcerated homicide offenders (in the
year 2000) from two Canadian federal institutions, one in British Columbia
on the west coast and one in Nova Scotia on the east coast. The inclusion
of offenders from two prisons allowed a large sample size and could
increase the generalizability of the findings. Mountain Institution is a
medium-security prison located in British Columbia that houses approximately
400 inmates at any given time. A review of file information
indicated that a total of 92 offenders had committed at least one homicide,
and extensive efforts were made to obtain adequate information on these
homicides to include them in our sample. There was detailed file information
on the homicide in 74 cases, which were included. The second prison
(Springhill Institution) is a medium-security prison located in Nova Scotia
also housing approximately 400 inmates. At the time of data collection,
there were 54 homicide offenders in this institution. Of these, detailed file
information regarding the homicide was available in 51 cases. Thus, in
total, there was detailed information on 125 homicide offenders.
PCL–R (Hare, 1991). The PCL–R has been widely adopted in the
assessment of psychopathy in forensic populations. Psychopathy, as measured
by the PCL–R, is characterized by 20 criteria, scored as 0, 1, or 2,
allowing a maximum score of 40. As recommended in the manual, a score
of 30 was the cut-off used for classifying psychopathy (Hare, 1991). The
PCL–R score is highly reliable over time and has demonstrated validity
according to a number of indices of validity (e.g., Fulero, 1995; Stone,
1995). Although there has been some debate over whether psychopathy
represents a discrete or a continuous variable (e.g., Harris, Rice, & Quinsey,
1994), recent research suggests that psychopathy may represent a
distinct clinical entity or taxon (see Hart & Hare, 1997). Nonetheless, we
used both a dichotomous and a continuous score approach to examine
psychopathy and homicide.
In the Canadian correctional system, risk assessments for the purposes of
conditional release and treatment programs normally include an evaluation
of psychopathy by a psychologist who has been well trained in the
administration of the PCL–R. PCL–R assessments are typically conducted
as part of the intake assessment and for conditional release decisions and
are based on a structured interview as well as a thorough review of all
collateral and historical information. A file search yielded all available
PCL–R scores and corresponding Factor 1 and Factor 2 scores as reported
in the official risk assessments. (It is now a requirement of the Canadian
correctional system that a risk assessment [including a PCL–R] be conducted
on all violent offenders.) However, due to the recentness of this
policy, 29 offenders from the Springhill sample still required a PCL–R
rating. Although PCL–R assessments are often based on a review of file
information and an interview with the offender, research (e.g., Grann,
Langstroem, Tengstroem, & Stalenheim, 1998; Wong, 1988) has consistently
shown that assessments based solely on the offender’s file information
are highly similar to ratings including an interview (see Hare, 1991)
and are appropriate in the absence of an interview (provided that there is
sufficient file information to code the PCL–R; files on Canadian federal
offenders are generally extensive, detailed, and multifaceted). For the
current study, a graduate student in psychology and a senior undergraduate
student who had been trained in administering the PCL–R reviewed all
available official file information and scored the 29 other PCL–Rs. These
raters were kept blind to the purpose and hypotheses of the study to prevent
any bias in their scoring of the PCL–R.
File documentation concerning the homicide. The crime information
was coded directly from the official Criminal Profile Reports (CPR) and
the Psychological Assessment Reports (PAR) included in the institutional
files. These two documents are considered to be the most important and
informative files within each offender’s case file for describing in detail the
offender’s violent crimes. The CPR is written by a case management
officer and is based on the official police report (submitted to the prosecutor
to allow charges to be laid) and court information (e.g., submissions
by the prosecutor). The report is an objective description of the actual
crime as a result of a thorough investigation and court testimony. Within
the CPR, an official, detailed description (typically 1–2 pages in length) is
provided for each serious offense. The PAR is a detailed assessment of the
offender’s psychological status and description of violent crimes, written
by a psychologist. The extensiveness of the documents contained within
each inmate’s institutional file clearly was sufficient for completing both
the homicide coding and the additional 29 PCL–Rs.
PCL–R reliability. Interrater reliability of the PCL–R assessments for
the entire sample was examined in two ways. First, 21 offenders (16.8%)
were randomly selected for dual coding. In these cases, with the exception
of the PAR documenting the original PCL–R score, all file information
available on each offender was made available to the blind coder. This
check ensured that the original PCL–R scores were accurate, as expected
given that the original raters were all highly trained psychologists. For a
second reliability check, a set of 33 cases was randomly selected for dual
coding. However, for these cases, all details pertaining to the current
homicide offense were completely removed prior to coding. Although this
practice for assessing reliability has rarely been adopted in psychopathy
studies, we felt that it was an important measure. The rationale for this
second test of reliability was that coding in the absence of the homicide
description would circumvent the possibility of “circularity” or contamination
in scoring the PCL–R. In other words, just as it was necessary for
homicide coding to be done without knowledge of whether the offender
was psychopathic (see below), we felt that it was important to demonstrate
that PCL–R scores were not unduly influenced by knowledge of the details
of the homicide.
Homicide coding scheme. The characteristics of the 125 homicides
were coded on the basis of pertinent information in the offenders’ files. To
avoid possible rating bias, the coder was kept unaware of the PCL–R score
(removed prior to coding). Overall, 13 of the 125 (10.4%) offenders had
committed more than one homicide; 11 offenders had committed two
homicides, and 2 offenders had committed three homicides. However, due
to a general lack of file information concerning “historical” homicides,
438 WOODWORTH AND PORTER
some of which were from decades earlier, only the most recent homicide
To examine in detailed (rather than dichotomous) fashion whether the
degree of instrumentality and reactivity associated with homicide was
associated with level of psychopathy, each homicide was rated on a
Likert-type scale with possible ratings ranging from 1 to 4. Based on
Cornell et al.’s (1996) coding criteria for instrumental and reactive aggression,
as well as an extensive literature review, this was conceptualized as
a continuum as follows:
1. Purely reactive: In order for a homicide to be rated as purely reactive,
there had to be strong evidence for a high level of spontaneity/impulsivity
and a lack of planning surrounding the commission of the offense. Reactive
violence was coded if there was evidence for spontaneity or impulsivity, a
rapid and powerful affective reaction prior to the act, and no apparent
external goal other than to harm the victim immediately following a
provocation/conflict. A clear example of a purely reactive homicide is if an
unknown victim verbally insulted the perpetrator, who in a rage immediately
started a fight and proceeded to stab the victim to death with a
weapon of “convenience” (e.g., a broken bottle in a bar).
2. Reactive/instrumental: To qualify for this rating, the homicide had to
show evidence for both reactive and instrumental violence. However, the
primary quality of the violence leading to death had to be reactivity. For
example, using the example above, the reactive/instrumental description
would apply if after or during the unplanned fight (and eventual murder),
the perpetrator elected to rob the victim as well. Thus, the evidence would
suggest that the homicide was unplanned/reactive but that there was also a
secondary instrumental, opportunistic component.
3. Instrumental/reactive: To qualify for this rating, the homicide had to
show evidence for both instrumental and reactive violence. However, the
primary quality of the violence leading to death had to be instrumental. For
example, an instrumental/reactive homicide would be coded if the offender
started to commit a bank robbery but in the process proceeded to murder
a bank teller after becoming agitated when the teller picked up a phone. In
this case, a crime occurred for an obvious external gain, and the homicide
was part of this instrumental act. However, the homicide occurred as a
reaction to unplanned events within the context of the crime.
4. Purely instrumental: For a homicide to be rated as purely instrumental,
the offense had to have been clearly goal-oriented in nature with no
evidence of an immediate emotional or situational provocation. The homicide
had to have been committed for a clearly identifiable purpose other
than “hot-blooded” spontaneous anger or a response to an immediate
frustration. Therefore, a purely instrumental homicide was coded if there
was strong evidence that the homicide had been intentional, premeditated
(nonimpulsive), motivated by a clear external goal such as drugs, money,
to obtain sex or revenge, and not immediately following a potent affective
reaction. For example, an offender may have carefully planned, carried out,
and concealed a homicide in order to steal from the victim. We also
examined this issue categorically to test whether the psychopaths and
nonpsychopaths had engaged in primarily reactive (rating of 1 or 2) or
instrumental violence (rating of 3 or 4).
Instrumental violence was then classified further according to the following
categories: primary instrumental violence and secondary instrumental
violence. Instrumental violence was identified as primary when its
main purpose was to inflict harm on an individual (e.g., revenge) and not
to serve some other purpose such as material gain (e.g., drugs, money). In
contrast, instrumental violence was considered secondary when the main
purpose was not to inflict pain on the victim but to achieve a clear goal
(e.g., drugs, money), and violence was committed only as a means by
which to achieve these goals. Indeed, it should be noted that although we
relied heavily on Cornell et al. ‘s (1996) original coding scheme, after an
extensive literature review we decided to include planned revenge/
retribution as a potential type of instrumental aggression. This was meant
to reflect the growing concern of researchers that instrumental violence,
although planned and nonimpulsive, sometimes is committed primarily for
the purpose of inflicting pain and harm on another person and that “hostile”
aggression also should sometimes be viewed as instrumental (e.g., Bushman
& Anderson, 2001; Indermaur, 1996; Tedeschi & Felson, 1994).
Therefore, if there was a “cooling off” period, or a discernible gap in time
between the provocation/frustration and the homicide, revenge/retribution
was coded as an instrumental motive. Further, the various motivations that
the offenders may have had for committing an instrumental homicide were
investigated. Specifically, the possibility that the instrumental violence had
been committed (a) for monetary gain, (b) drugs/alcohol, and/or (c) revenge/retribution,
(d) to obtain nonconsensual sex, or (e) for an idiosyncratic
reason was recorded. A homicide was not coded if the motive or
rationale for committing the homicide could not be determined or if it was
not possible to ascertain whether the homicide had been spontaneous or
As was conveyed in our main coding description above, the degree of
instrumentality or reactivity associated with a homicide mainly considered
(a) instrumental gain, (b) impulsivity, and (c) level of antecedent affective
arousal. We assumed that these dimensions would generally be closely
interrelated in considering homicidal violence. For example, we predicted
that a clear instrumental gain would generally be associated with low
impulsivity and low affective arousal. However, it was important to explore
empirically how these three main components co-occurred and how
each related to both the instrumentality ratings and the PCL–R scores.
Therefore, these three dimensions were coded for the entire sample of the
homicides (and dual coded for a reliability check in 19 [15.2%] cases) to
allow a careful delineation of their interrelationships and relationships with
instrumental violence and psychopathy. Trained raters coded for the presence
or absence of an instrumental gain (evidence or no evidence), impulsivity
(not, somewhat, or highly impulsive; Hare, 1991), and affect arousal
(low, moderate, or high amount of emotional arousal). (Interested readers
may contact the authors for more detailed information on how the three
dimensions were coded.)
PCL–R scores and interrater reliability. For the entire sample,
the mean PCL–R total score was 22.27 (SD 8.81; range
1–37). Using the diagnostic cut-off score of 30, offenders were
classified either as psychopaths or nonpsychopaths. Ninety-one
(72.8%) offenders scored below the cut-off and were classified as
nonpsychopaths, whereas 34 (27.2%) offenders scored within the
A preliminary interrater reliability check was conducted on the
PCL–R scores, using 21 (16.8%) randomly selected case files for
dual coding. Intraclass correlation coefficients (ICC) were examined
to determine the level of inter-rater reliability for continuous
scores. Interrater reliability was high/acceptable for PCL–R total,
Factor 1, and Factor 2 scores (ICCs .92, .81, and .95, respectively;
ps .001). Further, there was no mean difference between
the two sets of scores (M 24.95, SD 7.91, and M 25.81,
SD 6.91, for Rater 1 and Rater 2, respectively), t(40) .37, p
.05. Similarly, computing Cohen’s kappa revealed an acceptable
level of agreement between Raters 1 and 2 for classifying the
offenders as psychopaths or nonpsychopaths, kappa .79, p
.001 (common guidelines for acceptable kappa scores are .40
poor; .40–.59 fair; .60–.74 good; and .75 excellent; e.g.,
Cicchetti & Sparrow, 1981; Fleiss, 1981).
To examine the potential problem of circularity or criterion
contamination, a second interrater reliability check was conducted
on an additional 33 (26.4%) randomly selected files in which the
PSYCHOPATHY AND HOMICIDE 439
raters were kept blind to the description of the offenders’ current
homicide offense (the description of the offense was removed in
advance of coding). Similar to the initial reliability check, interrater
reliability was high/acceptable for PCL–R Total, Factor 1,
and Factor 2 (.97, .95, and .94, respectively; ps .001). This
analysis established that the PCL–R ratings were valid and argues
against the possibility of circularity in the ratings.
Interrater reliability for homicide coding. An interrater reliability
check was conducted on the variables coded pertaining to
the characteristics of the homicides. A second well-trained rater
who followed the same coding guidelines as the first rater coded
the homicide variables (the second rater had been trained over a
2-day period and was kept blind to the PCL–R data for each
offender). The reliability check (using 21 randomly selected files)
indicated that the coded scores were highly reliable. Specifically,
reliability was high/acceptable for type of homicide, K(21) .81,
p .001, and specific type of instrumental violence, K(21) .87,
p .001.1 An interrater reliability check also was conducted on
the three dimensions of the homicide offenses, using 19 randomly
selected case files. ICCs were used to determine the level of
interrater reliability for these scores. Inter-rater reliability was
high/acceptable for gain, impulsivity, and affect (ICCs .90, .95,
and .88, respectively; ps .001).
Age. The offenders’ mean age at the time of data collection
was 41.8 years (SD 10.5; range 18–67 years). The mean age
at the time they committed the current homicide was 30.0 years
(SD 9.5; range 14–55 years). The age at which the offender
committed the current homicide did not differ significantly between
psychopaths and nonpsychopaths ( p .05).
Characteristics of the victims. The victims’ mean age at time
of death (based on a subset of 68 victims for whom the specific age
at time of death was listed in the file information) was 31.6 years
(SD 9.5; range 3–92). The general age group of the victim
was reported in 100 cases. Eight victims were children (0–12 years
old; 8%), 15 were teenagers (15%), 69 were adults (20–64 years
old; 69%), and 8 were seniors (65 years and older; 8%). In
addition, in 8 cases (6.4%) the offender had more than one victim
during his current homicide offense.
Relationship Between Psychopathy and Homicide Offense
Instrumental/reactive differences as a function of psychopathy.
Overall, 45 (36%) homicides were purely instrumental, 25 (20%)
were instrumental/reactive, 29 (23.2%) were reactive/instrumental,
16 (12.8%) were purely reactive, and 10 (8%) could not be
coded. Possible differences in the violence committed by the
psychopathic and nonpsychopathic groups were examined.2 Results
indicated that there was a significant difference between the
two groups, t(113) 3.73, p .001, 2 .11.3 Specifically,
homicides perpetrated by psychopaths were associated with a
higher degree of instrumentality (M 3.47, SD .82) than
homicides committed by nonpsychopaths (M 2.65, SD 1.10).
There also was a significant correlation between the continuous
PCL–R total scores (0–40) and the instrumental ratings. Higher
scores on the PCL–R were associated with higher levels of instrumental
violence, r(115) .45, p .001.4 Psychopaths and nonpsychopaths
were then compared on whether their violence was
primarily reactive (rating of 1–2) or primarily instrumental (rating
of 3–4). Results indicated that, overall, 70 (60.9%) of the offenders
had committed a primarily instrumental homicide, whereas 45
(39.1%) offenders had committed a primarily reactive homicide.
Again, a significant relationship between type of homicide and
psychopathy was found. Specifically, psychopathic offenders were
far more likely, 2
(1, N 115) 17.96, p .001, to have used
primarily instrumental violence (93.3%), compared with nonpsychopathic
offenders who were more likely to have committed
primarily reactive rather than instrumental violence (51.6%; (see
Figure 1). It is interesting that nonpsychopathic offenders (48.4%)
also were clearly capable of committing primarily instrumental
homicides but to a much lesser extent than psychopaths.
An analysis of the three separate dimensions (affect, instrumental
gain, and impulsivity) revealed that although these dimensions
were partially interrelated, they each contributed in a meaningful
way to the instrumental/reactive coding scheme. As expected, gain
ratings were significantly negatively correlated with impulsivity
ratings, r(109) .62, p .001, and negatively (nonsignificantly)
correlated with ratings of affective arousal, r(48) .26,
p .078, whereas affect and impulsivity were positively correlated,
r(50) .59, p .001. Further, as expected, gain ratings
were significantly positively related to the instrumental/reactive
ratings, r(112) .63, p .001, whereas affect, r(50) .54, p
.001, and impulsivity, r(112) .84, p .001, were significantly
A hierarchical regression analysis was conducted with affect,
gain and impulsivity entered sequentially as potential predictors of
the instrumental/reactive ratings. The model was significant,
adjR2 .70, F(3, 44) 37.36, p .001. Specifically, when affect
was entered into the model, it significantly predicted instrumental/
reactive ratings, chR2 .27, F(1, 46) 16.87, p .001. Next, the
gain dimension was entered and was also found to add signifi-
1 Although kappa is most commonly used when comparing the reliability
of dichotomous variables, its suitability for a multileveled categorical
variable has also been shown to be appropriate (Carletta, 1995; Howell,
2 As mentioned in the introduction, from our theoretical framework we
conceptualized the ratings as representing a continuum. We also analyzed
the instrumental/reactive data categorically using a nonparametric approach
and obtained the same pattern of results.
3 When the 29 PCL–Rs that had been completed by the researchers (on
the basis of file information only) were excluded from this analysis, the
effect size was almost identical ( p .001, 2 .12), indicating the same
pattern of results as obtained with the full sample.
4 Previous literature has suggested that individuals who score above 20
on the PCL–R also display many of the characteristics of psychopathy,
although they are not formally labeled as psychopathic. Therefore, the
sample also was broken down into three PCL–R categories of low (0–20;
n 39), medium (20–30; n 46), and high (30–40; n 30) psychopathy.
Similar significant results were again obtained, F(2, 112) 16.32, p
.001. Specifically, the high-psychopathy group (M 3.47/4) committed
60% instrumental, 33.3% instrumental/reactive, 0% reactive/instrumental,
and 6.7% reactive homicides. The medium-psychopathy group (M
3.04/4) committed 43.5% instrumental, 23.9% instrumental/reactive, 26.1%
reactive/instrumental, and 6.5 % reactive homicides. The low-psychopathy
group (M 2.18/4) committed 17.9% instrumental, 10.3% instrumental/
reactive, 43.6% reactive/instrumental, and 28.2% reactive homicides.
440 WOODWORTH AND PORTER
cantly to the prediction model, chR2 .31, F(1, 45) 33.38, p
.001. Finally, when the impulsivity dimension was included in the
model, it also significantly added to the prediction model, chR2
.14, F(1, 44) 21.56, p .001. However, with all three variables
entered concurrently into the model, only the impulsivity dimension
uniquely predicted instrumental/reactive ratings, .65,
t(44) 4.64, p .001, whereas affect, .10, t(44) .93,
p .36, and gain ratings, .181, t(44) 1.52, p .14, did not
predict scores independently.
It also was important to examine how the three dimensions of
the homicide offense were related to the offender’s overall PCL–R
score. Results indicated that although the presence of external gain
was significantly positively related total PCL–R score, r(112)
.28, p .01, the presence of impulsivity was significantly negatively
related to total PCL–R score, r(112) .26, p .01.
Affective arousal was negatively related to total PCL–R score, but
the correlation was nonsignificant, r(52) .17, p .23.
To examine the relative contributions of Factor 1 (F1; interpersonal/affective
traits) and Factor 2 (F2; antisocial behavior) in
predicting the degree of instrumentality within the homicides, a
regression model consisting of F1 and F2 scores (entered simultaneously
into the model) was conducted. The regression equation
was significant, adjR2 .20, F(2, 112) 15.65, p .001.
Although the partial correlation for F1 scores was significant,
r(115) .37, p .001, the partial correlation for F2 scores was
not, r(115) .09, p .05, suggesting that F1 but not F2 scores
played a role in predicting the level of instrumentality in the crime.
These results were confirmed by a calculation of the zero-order
correlations between PCL–R total score, Factor 1, Factor 2, and the
type of homicide (see Table 1).
Figure 1. Primary nature of homicides as a function of psychopathy.
Zero-Order and Partial Correlations Between Factor 1, Factor 2, PCL–R Total,
and Homicide Type
Type of correlation Factor 1 Factor 2 PCL–R total Homicide type
Factor 1 — .53* .85* .46*
Factor 2 — .87* .31*
PCL–R total — .45*
Homicide type —
Partial correlation, with Factor 1 removed
Factor 2 — .09
PCL–R total — .13
Partial correlation, with Factor 2 removed
Factor 1 — .37*
PCL–R total — .39*
Note. PCL–R Psychopathy Checklist—Revised.
* p .001.
PSYCHOPATHY AND HOMICIDE 441
Psychopathy and victim gender. In 41.6% of the homicides,
the victim was male and in 54.4% cases the victim was female.
The relationship between psychopathy and victim gender was
(2, N 115) 6.94, p .05. When victim gender
was known, offenders in the nonpsychopathic group committed
homicides against males (48.4%) and females (47.3%) in approximately
equal numbers, whereas offenders in the psychopathic
group committed homicides more commonly against females
(73.5%) than males (23.5%).
Specific type of instrumental violence. Possible motives behind
the type of instrumental violence used were examined. In 16
cases, this variable was not coded because the homicides were
purely reactive with no instrumental component. Ten additional
cases were not coded either because of a lack of information.
Therefore, the sample size for this analysis was 99. Results indicated
that 30 homicides (30.3%) were committed for revenge or
retribution, 24 (22%) for monetary gain, 21 (19.3%) to obtain
nonconsensual sex, 11 (11.1%) occurred in a conflict over a
female, 7 (6.4%) were for other reasons, and 3 (2.8%) were to
obtain drugs or alcohol. No significant relation was found between
psychopathy and the specific type of instrumental violence
General type of instrumental violence. The final issue examined
was whether instrumental homicides showed primary instrumental
violence or secondary instrumental violence. Purely reactive
homicides or those that could not be coded because of
insufficient information were excluded from the analysis. In addition,
8 cases were excluded because it was unclear whether instrumental
violence should be coded as secondary, primary, or a
combination. Therefore, the sample size for this analysis was 97.
Results indicated that secondary instrumental violence had been
committed in 26 (26.8%) of the homicides, whereas primary
instrumental violence was perpetrated in 65 (67.0%) of the cases,
and 6 homicides (6.2%) showed a combination of both types of
instrumental violence. Thus, more than twice as many offenders
committed primary instrumental violence compared with secondary
instrumental violence. However, no significant relation was
found between psychopathy and the general type of instrumental
violence committed, 2
(2, N 91) .49, p .05.
Much research has established a strong connection between
psychopathy and criminal behavior, including violence (see Hart &
Hare, 1997). Psychopaths seem to have few inhibitions to prevent
callous interactions with others across the life span (e.g., Hare
1996, 1998; Harpur & Hare, 1994; Porter, Birt, & Boer, 2001;
Simourd & Hodge, 2000). However, the current study was the first
to examine the relationship between psychopathy and the most
serious form of crime—homicide. We predicted that psychopaths
would show a higher level of instrumentality in their homicides
than nonpsychopaths, who would be more likely to have committed
reactive crimes of passion. On the other hand, it is also possible
that given their expected pattern of impulsivity, psychopathic
offenders might have perpetrated spontaneous and reactive murders
(e.g., Hare, 1998).
The results clearly supported the hypothesis that psychopaths
are more likely to engage in instrumental or cold-blooded homicides
compared with nonpsychopathic individuals. In fact, almost
all of the psychopaths had committed a primarily instrumental
murder.5 Our data suggested that nonpsychopathic offenders were
certainly capable of committing instrumental offenses, but they did
not show the same clear preference for or tendency toward instrumental
violence witnessed in the psychopathic offenders. There are
a number of possible reasons for this finding. First, psychopathic
offenders characteristically show a marked lack of empathy toward
others (e.g., Levenston, Patrick, Bradley, & Lang, 2000), and this
appears to extend to their crime victims. Recent research by
Herpertz et al. (2001) indicated that psychopaths display a profound
level of hypoemotionality that could effectively disallow an
inhibition against acting in a violent manner if it served a selfish
function. Here, psychopaths appeared to be capable of premeditating
and carrying out ruthless, cold-blooded homicides that many
nonpsychopathic (although potentially violent) individuals would
be considerably less likely to consider perpetrating (also see Abbott,
2001). For example, one psychopathic offender (scoring at
the 87th percentile on the PCL–R relative to other inmates) admitted
to police that he had decided to murder an ex-girlfriend
because he felt that she was interfering with his new relationship,
and he simply decided that murdering her would help resolve this
issue. Another inmate carefully planned and murdered his wife
because he stood to gain financially from her insurance policy. It
is likely that few people without the affective deficit associated
with psychopathy would seriously consider such acts, and even
fewer would actually plan and carry them out. It is of note that
previous studies have found that an inability to experience or
anticipate the remorse (characteristic of psychopathy) that is often
a consequence of aggressive behavior may lead to an increase in
instrumental aggression (e.g., Guerra, Nucci, & Huesmann, 1994;
Kingsbury et al., 1997).
It was interesting that the overall level of instrumental violence
characterizing these homicides was substantially higher than expected
on the basis of previous contentions. However, although
there seems to have been a long-standing and widely held belief
that most homicides are reactive, emotional, or even irrational,
there were actually few empirical data to speak to the issue prior
to the current study. In fact, the current study was one of the first
to look specifically at the offense of homicide in terms of rich,
well-defined instrumental/reactive criteria. In our view, past conceptions
of homicide greatly underestimated the relevance of forethought
and instrumentality in understanding the phenomenon.
The majority of murderers in this study did not “snap” and kill
another person (although some did) as many might have expected.
In our view, more research is needed (perhaps by interviewing the
offenders themselves) to increase our understanding of why so
many homicide offenders “chose” to engage in this type of
Among the most important findings was that nearly all of the
psychopaths had perpetrated primarily instrumental homicidal vi-
5 Note that the data reported here do not allow a determination of
whether psychopaths are more likely than nonpsychopaths to commit an
instrumental homicide (or less likely to commit a reactive homicide)
because the prevalence of psychopathy in the general population is not
clear (despite a published estimate of 1%; e.g., Hare, 1996). Nonetheless,
among those who have committed a murder, nonpsychopaths are far more
likely to have committed a reactive murder.
442 WOODWORTH AND PORTER
olence and that so few had committed highly impulsive homicidal
violence. It has been long understood that individuals with psychopathic
qualities will sometimes engage in highly spontaneous,
impulsive behavior in criminal and noncriminal contexts (e.g.,
Ellis, 1987; Hare, 1996). As noted by Newman and Schmitt
(1998), “[P]sychopathic individuals are notorious for their failure
to inhibit or modify behaviors that culminate in negative consequences”
(p. 527). Given this connection with impulsivity and lack
of behavioral controls, why were the homicide offenders in the
current study unlikely to have engaged in primarily impulsive,
reactive violence? We think that this pattern could possibly reflect
selective impulsivity; that is, psychopaths may behave in a more
instrumental manner (or, rather, may behave in a less reactive and
impulsive manner) specifically for the offense of homicide. It is
possible that when committing an act with such extreme negative
consequences as with homicide perpetration (e.g., lifetime incarceration),
psychopaths may plan their actions in a calculating
fashion because the stakes are high. Although it may seem somewhat
paradoxical that psychopaths would still elect to murder
someone after a more rational appraisal of the potentially serious
costs of perpetrating the act, this process might be influenced by an
undersensitive behavioral inhibition system (BIS). Some researchers
have suggested that the BIS is weaker in psychopaths who are
seemingly unable to properly inhibit their behavior even when
presented with serious punishment cues (e.g., Fowles, 1980).
Psychopathic offenders also might be more likely than other
offenders to resist an impulse to kill someone when caught in an
emotion-driven dispute or less likely to experience such powerful
emotions in the first place. Thus, the impulsive behavior often seen
in psychopaths outside of the context of homicide may not be
simply uncontrollable or reflect an inability to consider the consequences
but rather may be a function of not caring to control or
inhibit the behavior. In fact, our results indicated that of the three
dimensions of instrumentality we considered, impulsivity contributed
most to the variance of the instrumental/reactive scores and
was, surprisingly, negatively related to the overall PCL–R score in
these homicide offenders. It is clear that this issue must be investigated
in future research before solid conclusions can be
The results indicate that the PCL–R factor scores were differentially
related to the instrumentality of the homicides. Specifically,
Factor 1 scores accounted for much of the variance associated
with the instrumentality of the homicides, whereas Factor 2
scores did not significantly contribute to this dimension. These
findings appear to be consistent with Dempster et al.’s (1996)
study of 75 adult male violent offenders attending an inpatient
treatment program for violent offenders. They found that Factor 1
was significantly related to ratings of planning and instrumentality,
whereas Factor 2 actually had a negative relationship with elements
of planning during the offense (see also Cunningham &
Reidy, 1998; Patrick & Zempolich, 1998).
This study had many strengths in addition to being the first to
examine the relationship between psychopathy and homicide. Our
results supported the contention of researchers such as Bushman
and Anderson (2001) that many acts of aggression cannot be
categorized as strictly instrumental or reactive but, rather, contain
elements of both. Further, we had access to a large sample to study
a poorly understood and highly consequential form of abnormal
behavior, devised a highly reliable and rich coding scheme for
characterizing instrumentality and its basic elements, and ensured
that no circularity problem was present.
Future studies could explore consistencies (or inconsistencies)
between the offender’s primary motivation (instrumental or reactive)
for previous homicides and other violent acts and his or her
motivation for the current homicide. Research examining whether
these results would generalize across a range of criminal offenses
would be useful in testing the validity of our selective impulsivity
hypothesis (e.g., examining whether psychopaths who had committed
instrumental homicide also had committed reactive, nonhomicidal
violence). It is also possible that research examining
particular groups or subcultures could obtain different results. For
example, research on homicides committed by inner-city gangs,
organized criminals, or terrorists could yield different results regarding
both the type of homicides committed and the perpetrators’
In conclusion, we carefully investigated the psychological aspects
of homicide—a type of violent behavior that often seems
incomprehensible to both the public and mental health professionals—and
found that the construct of psychopathy contributes much
to our understanding of the phenomenon. Psychopaths engage in
far more instrumental or cold-blooded homicides than other offenders.
Given the wealth of information now available on the
behavioral and personality patterns seen in psychopaths across the
life span (e.g., Porter, Birt, & Boer, 2001), the homicide investigator
could potentially reduce the field of suspects in difficult
investigations. In terms of treatment planning in the prison setting,
it seems clear that a consideration of psychopathy and the type of
violence committed is necessary (e.g., anger management would
not seem to be an optimal approach for the psychopathic murderer).
Future research should attempt to differentiate nonpsychopathic
offenders (nearly half in this study) who commit primarily
instrumental homicides from those who commit primarily reactive
homicides. Further, in light of these results, classic conceptions of
impulsivity in psychopaths may need to be reconsidered. As we
have argued, it may be that “impulsivity” in psychopaths has less
to do with a lack of control than with conscious decision making
that depends on a rapid consideration of the gravity of the
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Received May 7, 2001
Revision received October 24, 2001
Accepted December 18, 2001