E>


1
To Appear in P. Carruthers, S. Laurence and S. Stich (Eds.), The structure of the
innate mind. New York: Oxford University Press. [Final version delivered April
16, 2004.]

17 The Plausibility of Adaptations for Homicide


Joshua D. Duntley and David M. Buss


A partner of mine said he might come over to my pad with some broads, so I
hurried over to the liquor store right around the corner to get a case of beer.
As I was walking across the parking lot of the store, this guy almost ran me
over. I flipped him off. The driver and his partners jumped out of the car and
rat-packed me. They knocked me down, and the driver pushed my head into
the dirt next to the cigarette butts. Then they went into the store. I just felt,
What a low fucking thing to do to somebody. They are just a bunch of yellow
motherfuckers. In my mind I suddenly thought, Ive got to get back at these
dirty motherfuckers, and I ran back to my pad for my rifle.

I got back to the liquor store as fast as I possibly could and waited for them
about twenty yards from the front door of the store. Finally his two partners
popped out the door. I said to myself, Fuck it, Ill shoot all of them. I fired
two quick, wild shots but missed them both, and they got away. I decided then
that I better put the barrel to the chest of the motherfucker who I really
wantedthe driverand make sure that I didnt miss him. I had stone hatred
for him, and I righteously couldnt wait to see the look on his face when I blew
him away. As soon as he popped out of the liquor store door, I charged right
up to him, rammed the barrel in his chest, and pulled the trigger.
(Athens,
1997
p.
10)

1 Introduction
People kill other people in every known culture around the world. The question is
why. This chapter presents a new theory of homicide, Homicide Adaptation Theory,
which proposes that humans evolved adaptations to facilitate killing. The new theory
is contrasted with two competing conceptions of why people kill: The Byproduct
Hypothesis and the Evolved Goal Hypothesis. Prior to presenting these competing
views of homicide, we discuss the concept of innateness in relation to our
conception of evolved homicide adaptations.

2
The Concept of Innateness from the perspective of Evolutionary
Psychology
The term innateness is used to refer to many different phenomena (see Elman,
Johnson, & Bates, 1996). Our conceptualization of innateness falls in line with the
standard definition used by evolutionary psychologists and biologists when referring
to any adaptation. Selection has shaped the genes that pattern human ontogeny.
These genes provide the blueprint for the development of adaptations. Like the
blueprints for a house, they rely on resources and information present in the
environment to construct the adaptations for which they code. These features of the
environment were presumably recurrent in all or most generations of individuals in

2
the evolutionary history of an adaptation in order for selection to have made them an
integral part of reliable adaptation development.
Tooby and Cosmides (1992) refer to the statistical composite of selection
pressures that shaped an adaptation as its environment of evolutionary adaptedness or
EEA. Different sets of selection pressures contributed to the evolution of every,
individual adaptation, tailoring each to have a specific function in contributing to the
solution of a specific problem of survival and reproduction. Thus, each adaptation has
its own unique history of selection pressures and therefore its own unique EEA.
The function of a given adaptation can be affected by recurrent adaptive
problems in three primary ways. First, by their presence or absence, characteristics of
the environment can determine whether or not an adaptation develops at all. Take, for
example, the visual system. Forced to live in an environment without any visual light
from the time a person was born until adolescence, his visual system would not
develop normally. If he was suddenly exposed to visual light during adulthood, he
would have difficulty focusing his eyes, distinguishing between objects, and orienting
himself with visual cues (Sacks, 1995). The human visual system evolved in ancestral
environments where visual light was a recurrent feature and depends on the presence
of this environmental feature in abundance for its reliable development.
Second, the presence, absence, or amount of a feature of the environment may
contribute to the developmental trajectory of an adaptation. At certain points in
peoples lifetime, particularly during childhood, individuals come to a developmental
fork in the road. The contingency of environmental features that they face or have
faced thus far in their development determines in large part their future developmental
trajectory. Belsky, Steinberg, and Draper (1991), for example, argue that pubertal
onset and patterns of adult sexual behavior are influenced by father presence or
absence in the home. Their research findings suggest that, among female offspring,
father absence is associated with earlier onset of menarche, earlier first intercourse,
and a greater number of sexual partners. This pattern is proposed to be the result of
adaptations fashioned to recognize that there is a low probability of reliable male
investment in reproduction. Such psychological adaptations are argued to function
outside of conscious awareness.
Third, adaptations can be designed by selection to be prepared with different
adaptive contingencies in different environments. As situations change, one adaptive
contingency may be reversed or abandoned in favor of a different contingency. For
example, the skin, like any organ, is vulnerable to injury. Depending on the kinds of
tasks in which an individual routinely engages, some areas of the skin may be more
likely to be injured than others. As a protective measure, callous production has
evolved as a defense mechanism against repeated friction, preventing injury to the
skin (Buss, 2004). Callous production is an adaptive contingency that is active only
in response to specific environmental inputs (repeated friction to the skin). When the
friction disappears, callous production may stop as well.
Each of these examples describes an innate adaptation. They are evolved,
functional solutions to adaptive problems that reliably develop in normal
environments. They evolved in response to recurrent contexts of ancestral
environments and require the presence of the same features to develop and function
normally. The conceptualization of innateness explained in this section forms the
foundation of our hypotheses about adaptations for homicide.

3
Adaptations for Homicide

3
We propose that humans possess adaptations that evolved to produce homicide (Buss
& Duntley, 1998; Buss & Duntley, 2003; Buss & Duntley, 2004). Psychological
adaptations for homicide were selected when they contributed to better fitness
outcomes, on average, than competing designs present in the population at the time.
Certain information processing adaptations in our brains were shaped by selection
specifically to scrutinize and sometimes produce homicidal behavior when an
individual faces an adaptive problem similar to one recurrently solvable by homicide
in the past. In this chapter, we will (1) discuss our theory that humans evolved
adaptations for homicide, (2) discuss two alternative evolutionary theories of
homicide, and (3) review relevant homicide data that will help us to evaluate the
plausibility of our theory and the other theories of homicide.

3.1
The Nature of Selection Pressures for Homicide Adaptations
A description of adaptations for homicide begins with the recurrent adaptive problems
they evolved to solve. We hypothesize that a combination of simultaneously relevant
contextual factors, not any one single factor, acted as selection pressures that shaped
psychological adaptations for homicide. Therefore, it is not possible to point to just
one feature of a context that will activate a psychology of homicide in every instance,
in every person. There are always other, mitigating environmental factors present in
any real world situation that were also part of the overall selection pressures that
shaped homicide adaptations. In other words, any set of contextual cues to an
adaptive problem that was ancestrally solvable by homicide is made up of multiple
inputs. The presence or absence, as well as the severity of inputs demonstrated to
contribute to the activation of homicide adaptations can help us to predict when
homicide will be more or less likely to occur. Homicidal behavior is not under the
control of a simple ONOFF switch that can be manipulated with a push from a
single factor. The activation of evolved psychological mechanisms for homicide
requires the presence of co-occurring sets of circumstances, made up of factors such
as: (1) the degree of genetic relatedness between killer and victim, (2) the relative
status of the killer and victim, (3) the sex of killer and victim, (4) the size and strength
of the killers and victims families and social allies, and (5) the relative reproductive
values of the killer and victim.

3.2
Recurrent Adaptive Problems Solvable by Homicide
Homicide could not have evolved as a strategy unless it was ancestrally associated
with greater reproductive success than competin