Evolutionary Psychology of Homicide
David F. Bjorklund
Florida Atlantic University
Evolutionary psychology
 Understanding the human mind and behavior from
an evolutionary perspective.
Basic tenet of evolutionary psychology:
 The human mind has been prepared by natural
selection, operating over geological time, for life
in a human group.
Nothing in biology makes sense except in the
light of evolution.
Theodosius Dobzhansky
Nothing in psychology makes sense except
in the light of evolution.
Every evolutionary psychologist
Darwin’s (1859) theory of natural selection
 Superfecundity
 Variation is characteristic
among members of a species
 Variation is heritable
 Characteristics that result in
individuals surviving and
reproducing tend to be selected
as a result of the interaction
between the individual and the
environment
The key to natural selection:
 Differential
reproductive success
because of heritable
variants; everyone has
ancestors, but not
everyone leaves
descendants
Some Assumptions of Evolutionary Psychology
 What evolved are psychological mechanisms (information processing
mechanisms are the “missing link” in evolutionary explication)
 Evolved psychological mechanisms are instantiated in the brain.
 Psychological mechanisms evolved in the “environment of
evolutionary adaptedness” (EEA)
 Evolved psychological mechanisms are functional: Designed to solve
statistically recurrent adaptive problems
Adaptation: Evolved solution to specific problem
that contributes directly or indirectly to
successful reproduction.
How to identify adaptations?
 Reliability
 Efficiency
 Economy
 Improbable Usefulness
Products of natural selection
 Adaptations (umbilical cord)
 Inherited
 Develop reliably
 Solve problems of survival or reproduction
 Most are species-typical
 Byproducts of adaptations—no functional design (e.g.,
belly button)
 Noise—random effects, independent of adaptations (e.g.,
“iny” vs. “outy”)
A Functional Analysis
Pregnancy Sickness as an Adaptation
Might there be some
adaptive value to
pregnancy sickness?
(e.g., Flaxman &
Sherman, 2000;
Profet, 1992)
 Symptoms: nausea,
vomiting, food
aversions
It corresponds to the time in prenatal development
when embryo is most susceptible to effects of
teratogens
Modern women acquire aversions to foods that are
high in toxins (e.g., meat, coffee, alcohol)
 Despite variability, pregnancy sickness in
universal
 Reduction in incidence of spontaneous abortions
for women who experience pregnancy sickness
Thalidomide babies
Naturalistic fallacy
 The false belief that because something is “
natural (“in the genes,” it “evolved”) it is good or
proper for the species.
Parental Investment Theory (Robert Trivers,
1972)





There is a conflict for both males and females in how much time, effort, and
resources to invest in mating versus parenting
In most mammalian species:
Females invest more than males (female investment is obligatory)
 Gamete size (egg larger than sperm)
 Internal fertilization and gestation
 Nursing
 Childcare
Ancestral men and women faced different adaptive problems and evolved
different adaptive mechanisms (this is true for the sexes of most animals)
The least investing sex (males) compete with one another for access to more
investing sex (females)
The Calculus of Maternal Investment
 Mothers will invest most in children who have the greatest
chance of reaching reproductive age, and thus carrying
forth the mother’s genes
 Mothers who are adept at identifying cues to a child’s
future reproductive success are more likely to invest the
most time, energy, and resources in those children
Factors Influencing amount of maternal
investment





Child’s health
Child’s age
Mother’s reproductive status
Social support
Parent-child conflict
Reduced maternal care can take many forms
 Neglect/abuse
 Receiving less attention, medical care, and food than they
might need
 Fostering out the child with relatives
 Oblation (leaving the child in the custody of some religious
institution)
 Abandonment
 Infanticide/Filicide
Children’s risk of homicide by a natural parent in relation
to age (from Daly & Wilson, 1988)
Homicides per million children
40
35
Nonrelatives
Natural Parent
30
25
20
15
10
5
0
0
1-4
5-8
9-12
Child Age
13-16
17
Risk of infanticide at hands of natural mother within first year
of life as a function of maternal age
Risk of infanticide as a function of mother’s age and
marital status
“Wicked” stepparents?
 Daly & Wilson, in survey of Canadian households: Children were 40
times more likely to be abused if they lived with a stepparent versus
two natural parents.
 This difference remained even when possible influencing factors that
may be associated with stepfamilies, such as poverty, the mother’s age,
and family size, were statistically controlled. Given these and similar
findings
 “Stepparenthood per se remains the single most powerful risk factor
for child abuse that has yet been identified.”
 Child homicide: Studies in different countries report that for children
under 2-years of age, homicide 40 to 100 times more likely at hands of
stepparent (usually stepfather) than natural parent.
Homicide Rate per Million Children
The risk of being killed by a stepparent versus a natural
parent in relation to child’s age: Canada, 1974-1983 (from
Day & Wilson, 1988)
650
600
550
500
450
400
350
Natural Parents
Stepparents
300
250
200
150
100
50
0
0-2
3-5
6-8
Age of Child
9-17
Young-male syndrome
 Males, at all ages, engage in more physical aggression than females
and the aggression that adolescent and young-adult males engage in is
more likely to lead to serious injury and sometimes death
 Greater male aggression can be explained in terms of parental
investment theory
Intrasex competition
Impress females via competitive risk taking
 Risk taking and accidents are frequently the result of competitive or
“show-off” behavior, with the purpose being to compete with other
members of the same sex or to impress members of the opposite sex
Death rates dues to vehicular accidents per 100,000
resident population in the United States for males and
females, 1995-1997
50
Veh icu lar Acciden t Rate
40
30
Male s
Fe male s
20
10
0
5-1 4
15 -24
25 -34
Ye ars i n Ag e
35 -44
45 -64
Percentage of trauma admissions for violence at LA Medical
Center for 3 months in 1990, by age (adapted from Cairns et
al., 1991)
Percentage of Violent Admissions
80
60
40
20
0
0-4
5-9
10-14
15-19
Age in Years
20-24
25-29
30-34
Same-Sex Homicides in Different Cultures
Homicide Victimization Rates by Age and Sex
for the United States in 1975
Homicide victimization rate per 100,000 resident
population in the United States, 1995-1997: Males
150
Homicide Rate / 100,000
130
110
All Males
White Males
African American Males
90
70
50
30
10
-10
1-14
15-24
25-44
Age in Years
45-64
65 +
Homicide victimization rate per 100,000 resident
population in the United States, 1995-1997: Females
Homicide Rate / 100,000
100
All Females
White Females
African American Females
80
60
40
20
0
1-14
15-24
25-44
45-64
Years in Age
65 +
Frequencies of Homicidal Fantasies for Men
and Women
Likelihood of Killing the Rival if the Crime
were not Discovered for Men and Women
Status by Warrior
Relationship between childhood maltreatment (none,
probable, severe) and MAOA activity (low versus high) on
antisocial behavior (from Caspi et al., 2002).
Composite Index of Antisocial
Behavior
1.25
1
Low MAOA
Activity
High MAOA
Activity
0.75
0.5
0.25
0
-0.25
-0.5
None
Probable
Severe
Childhood Maltreatment