Evolutionary Psychology of Homicide David F. Bjorklund Florida Atlantic University 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  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) (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)

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.

The risk of being killed by a stepparent versus a natural parent in relation to child’s age: Canada, (from Day & Wilson, 1988)

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,

Percentage of trauma admissions for violence at LA Medical Center for 3 months in 1990, by age (adapted from Cairns et al., 1991)

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, : Males

Homicide victimization rate per 100,000 resident population in the United States, : Females

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).