evolutionary psychology of homicide david f. bjorklund florida atlantic university david f....
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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