Evolutionary Psychology of HomicideEvolutionary Psychology of Homicide

David F. Bjorklund

Florida Atlantic University

David F. Bjorklund

Florida Atlantic University

Evolutionary psychologyEvolutionary psychology

Understanding the human mind and behavior from an evolutionary perspective.

Understanding the human mind and behavior from an evolutionary perspective.

Basic tenet of evolutionary psychology:Basic tenet of evolutionary psychology:

The human mind has been prepared by natural selection, operating over geological time, for life in a human group.

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 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

Nothing in psychology makes sense except in the light of evolution.

Every evolutionary psychologist

Darwin’s (1859) theory of natural selectionDarwin’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

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:The key to natural selection:The key to natural selection:The key to natural selection:

Differential Differential reproductive success reproductive success because of heritable because of heritable variantsvariants; everyone has ancestors, but not everyone leaves descendants

Differential Differential reproductive success reproductive success because of heritable because of heritable variantsvariants; everyone has ancestors, but not everyone leaves descendants

Some Assumptions of Evolutionary PsychologySome 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

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.

Adaptation: Evolved solution to specific problem that contributes directly or indirectly to successful reproduction.

How to identify adaptations? Reliability Efficiency Economy Improbable Usefulness

How to identify adaptations? Reliability Efficiency Economy Improbable Usefulness

Products of natural selectionProducts 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”)

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 AnalysisA Functional Analysis

Pregnancy Sickness as an AdaptationPregnancy Sickness as an Adaptation

A Functional AnalysisA Functional Analysis

Pregnancy Sickness as an AdaptationPregnancy Sickness as an Adaptation

Might there be some Might there be some adaptive value to adaptive value to pregnancy sickness?pregnancy sickness?

(e.g., Flaxman & (e.g., Flaxman & Sherman, 2000; Sherman, 2000; Profet, 1992)Profet, 1992)

Might there be some Might there be some adaptive value to adaptive value to pregnancy sickness?pregnancy sickness?

(e.g., Flaxman & (e.g., Flaxman & Sherman, 2000; Sherman, 2000; Profet, 1992)Profet, 1992)

Symptoms: nausea, Symptoms: nausea, vomiting, food vomiting, food aversionsaversions

Symptoms: nausea, Symptoms: nausea, vomiting, food vomiting, food aversionsaversions

It corresponds to the time in prenatal development It corresponds to the time in prenatal development when embryo is most susceptible to effects of when embryo is most susceptible to effects of

teratogensteratogens

It corresponds to the time in prenatal development It corresponds to the time in prenatal development when embryo is most susceptible to effects of when embryo is most susceptible to effects of

teratogensteratogens

Modern women acquire aversions to foods that are Modern women acquire aversions to foods that are high in toxins (e.g., meat, coffee, alcohol)high in toxins (e.g., meat, coffee, alcohol)

Modern women acquire aversions to foods that are Modern women acquire aversions to foods that are high in toxins (e.g., meat, coffee, alcohol)high in toxins (e.g., meat, coffee, alcohol)

Despite variability, pregnancy sickness in Despite variability, pregnancy sickness in universaluniversal

Reduction in incidence of spontaneous abortions Reduction in incidence of spontaneous abortions for women who experience pregnancy sicknessfor women who experience pregnancy sickness

Despite variability, pregnancy sickness in Despite variability, pregnancy sickness in universaluniversal

Reduction in incidence of spontaneous abortions Reduction in incidence of spontaneous abortions for women who experience pregnancy sicknessfor women who experience pregnancy sickness

Thalidomide babiesThalidomide babiesThalidomide babiesThalidomide babies

Naturalistic fallacyNaturalistic fallacy

The false belief that because something is “ natural (“in the genes,” it “evolved”) it is good or proper for the species.

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)

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)

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 InvestmentThe Calculus of Maternal InvestmentThe Calculus of Maternal InvestmentThe Calculus of Maternal Investment

Mothers will invest most in children who have the greatest Mothers will invest most in children who have the greatest chance of reaching reproductive age, and thus carrying chance of reaching reproductive age, and thus carrying forth the mother’s genesforth the mother’s genes

Mothers who are adept at identifying cues to a child’s Mothers who are adept at identifying cues to a child’s future reproductive success are more likely to invest the future reproductive success are more likely to invest the most time, energy, and resources in those childrenmost time, energy, and resources in those children

Mothers will invest most in children who have the greatest Mothers will invest most in children who have the greatest chance of reaching reproductive age, and thus carrying chance of reaching reproductive age, and thus carrying forth the mother’s genesforth the mother’s genes

Mothers who are adept at identifying cues to a child’s Mothers who are adept at identifying cues to a child’s future reproductive success are more likely to invest the future reproductive success are more likely to invest the most time, energy, and resources in those childrenmost time, energy, and resources in those children

Factors Influencing amount of maternal Factors Influencing amount of maternal investmentinvestment

Factors Influencing amount of maternal Factors Influencing amount of maternal investmentinvestment

Child’s healthChild’s health Child’s ageChild’s age Mother’s reproductive statusMother’s reproductive status Social supportSocial support Parent-child conflictParent-child conflict

Child’s healthChild’s health Child’s ageChild’s age Mother’s reproductive statusMother’s reproductive status Social supportSocial support Parent-child conflictParent-child conflict

Reduced maternal care can take many formsReduced maternal care can take many formsReduced maternal care can take many formsReduced maternal care can take many forms

Neglect/abuseNeglect/abuse Receiving less attention, medical care, and food than they Receiving less attention, medical care, and food than they

might needmight need Fostering out the child with relativesFostering out the child with relatives Oblation (leaving the child in the custody of some religious Oblation (leaving the child in the custody of some religious

institution)institution) AbandonmentAbandonment

Infanticide/FilicideInfanticide/Filicide

Neglect/abuseNeglect/abuse Receiving less attention, medical care, and food than they Receiving less attention, medical care, and food than they

might needmight need Fostering out the child with relativesFostering out the child with relatives Oblation (leaving the child in the custody of some religious Oblation (leaving the child in the custody of some religious

institution)institution) AbandonmentAbandonment

Infanticide/FilicideInfanticide/Filicide

Children’s risk of homicide by a natural parent in relation to age (from Daly & Wilson, 1988)

Children’s risk of homicide by a natural parent in relation to age (from Daly & Wilson, 1988)

0

5

10

15

20

25

30

35

40

0 1-4 5-8 9-12 13-16 17

Child Age

Homicides per million children

Nonrelatives

Natural Parent

0

5

10

15

20

25

30

35

40

0 1-4 5-8 9-12 13-16 17

Child Age

Homicides per million children

Nonrelatives

Natural Parent

Risk of infanticide at hands of natural mother within first year of life as a function of maternal age

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

Risk of infanticide as a function of mother’s age and marital status

““Wicked” stepparents?Wicked” stepparents?““Wicked” stepparents?Wicked” stepparents?

Daly & Wilson, in survey of Canadian households: Children were 40 Daly & Wilson, in survey of Canadian households: Children were 40 times more likely to be abused if they lived with a stepparent versus times more likely to be abused if they lived with a stepparent versus two natural parents.two natural parents.

This difference remained even when possible influencing factors that This difference remained even when possible influencing factors that may be associated with stepfamilies, such as poverty, the mother’s may be associated with stepfamilies, such as poverty, the mother’s age, and family size, were statistically controlled. Given these and age, and family size, were statistically controlled. Given these and similar findingssimilar findings

““Stepparenthood per se remains the single most powerful risk factor Stepparenthood per se remains the single most powerful risk factor for child abuse that has yet been identified.”for child abuse that has yet been identified.”

Child homicide: Studies in different countries report that for children 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 under 2-years of age, homicide 40 to 100 times more likely at hands of stepparent (usually stepfather) than natural parent.stepparent (usually stepfather) than natural parent.

Daly & Wilson, in survey of Canadian households: Children were 40 Daly & Wilson, in survey of Canadian households: Children were 40 times more likely to be abused if they lived with a stepparent versus times more likely to be abused if they lived with a stepparent versus two natural parents.two natural parents.

This difference remained even when possible influencing factors that This difference remained even when possible influencing factors that may be associated with stepfamilies, such as poverty, the mother’s may be associated with stepfamilies, such as poverty, the mother’s age, and family size, were statistically controlled. Given these and age, and family size, were statistically controlled. Given these and similar findingssimilar findings

““Stepparenthood per se remains the single most powerful risk factor Stepparenthood per se remains the single most powerful risk factor for child abuse that has yet been identified.”for child abuse that has yet been identified.”

Child homicide: Studies in different countries report that for children 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 under 2-years of age, homicide 40 to 100 times more likely at hands of stepparent (usually stepfather) than natural parent.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, 1974-1983 (from

Day & Wilson, 1988)

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)

0

50

100

150

200

250

300

350

400

450

500

550

600

650

0-2 3-5 6-8 9-17

Age of Child

Homicide Rate per Million Children

Natural Parents

Stepparents

0

50

100

150

200

250

300

350

400

450

500

550

600

650

0-2 3-5 6-8 9-17

Age of Child

Homicide Rate per Million Children

Natural Parents

Stepparents

Young-male syndromeYoung-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 competitionImpress 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

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 competitionImpress 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

Death rates dues to vehicular accidents per 100,000 resident population in the United States for males and

females, 1995-1997

0

10

20

30

40

50

5-14 15-24 25-34 35-44 45-64

Years in Age

Vehicular Accident Rate

Males

Females

0

10

20

30

40

50

5-14 15-24 25-34 35-44 45-64

Years in Age

Vehicular Accident Rate

Males

Females

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 trauma admissions for violence at LA Medical Center for 3 months in 1990, by age (adapted from Cairns et

al., 1991)

0

20

40

60

80

0-4 5-9 10-14 15-19 20-24 25-29 30-34

Age in Years

Percentage of Violent Admissions

0

20

40

60

80

0-4 5-9 10-14 15-19 20-24 25-29 30-34

Age in Years

Percentage of Violent Admissions

Same-Sex Homicides in Different CulturesSame-Sex Homicides in Different Cultures

Homicide Victimization Rates by Age and Sex for the United States in 1975

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: MalesHomicide victimization rate per 100,000 resident population in the United States, 1995-1997: Males

-10

10

30

50

70

90

110

130

150

1-14 15-24 25-44 45-64 65 +

Age in Years

Homicide Rate / 100,000

All Males

White Males

African American Males

-10

10

30

50

70

90

110

130

150

1-14 15-24 25-44 45-64 65 +

Age in Years

Homicide Rate / 100,000

All Males

White Males

African American Males

Homicide victimization rate per 100,000 resident population in the United States, 1995-1997: Females

Homicide victimization rate per 100,000 resident population in the United States, 1995-1997: Females

0

20

40

60

80

100

1-14 15-24 25-44 45-64 65 +

Years in Age

Homicide Rate / 100,000

All Females

White Females

African American Females

0

20

40

60

80

100

1-14 15-24 25-44 45-64 65 +

Years in Age

Homicide Rate / 100,000

All Females

White Females

African American Females

Frequencies of Homicidal Fantasies for Men and Women

Frequencies of Homicidal Fantasies for Men and Women

Likelihood of Killing the Rival if the Crime were not Discovered for Men and Women

Likelihood of Killing the Rival if the Crime were not Discovered for Men and Women

Status by WarriorStatus by Warrior

Relationship between childhood maltreatment (none, probable, severe) and MAOA activity (low versus high) on

antisocial behavior (from Caspi et al., 2002).

Relationship between childhood maltreatment (none, probable, severe) and MAOA activity (low versus high) on

antisocial behavior (from Caspi et al., 2002).

-0.5

-0.25

0

0.25

0.5

0.75

1

1.25

None Probable Severe

Childhood Maltreatment

Composite Index of Antisocial

Behavior

Low MAOAActivityHigh MAOAActivity

-0.5

-0.25

0

0.25

0.5

0.75

1

1.25

None Probable Severe

Childhood Maltreatment

Composite Index of Antisocial

Behavior

Low MAOAActivityHigh MAOAActivity