Transcript HOMICIDE An evolutionary perspective

Evolutionary forensic psychology perspectives
Presented By: Joseph A. Camilleri
Evolutionary Psychology
November 8th, 2002
Definitions
• Homicide
– Killing of one human being by another
• Neonaticide
– Killing of an infant within 24 hours after his/her birth
• Infanticide
– Killing of an infant within 1 year after his/her birth
• Filicide
– Killing of a son/daughter over the age of 1
• Uxoricide
– Killing of the wife by a husband
• Parricide
– Killing one’s parent
Why study homicide?
• Conflict assay
(Daly & Wilson)
– Window on evolved motivations that are
rarely lethal
• Severe and genuine conflict
– Unlike self-report
• Minimal biases of detection & reporting
Adaptive or Non-Adaptive Byproduct?
• Homicide is rare
• Non-Adaptive Byproduct
1. Male competition
2. Discriminative parental solicitude
3. Sexual jealousy/proprietariness
Who are the victims of homicide?
13th Century England
Homicide Data
Victims are rarely related
to the offender
Co-offenders are more
likely to be genetically
related
(Daly & Wilson, 1988)
Cross-cultural data
Victim-Offender
Co-offenders
"r"
N
"r"
N
Detroit
.03
508
.09
43
Miami
.01
494
.09
27
Bison-Horn
Maria
.09
130
.16
17
Bhil
.05
100
.27
22
Munda
.07
47
.33
9
Oraon
.06
43
.23
7
Tzeltal Mayans
.08
26
.35
6
Gros Ventre
.01
14
.50
1
13th C England
.01
2434
.08
2372
Daly & Wilson (1988) Homicide
Who does the killing?
Canada 1974-1990
England & Wales
Males
3881
3087
Females
94
108
% Male
98
97
1977-1990
Iceland 1946-1970
10
0
100
Chicago 1965-1989
Detroit 1972
Miami 1980
!Kung San
9761
282
359
19
229
9
0
0
98
97
97
100
93
33
1
0
99
100
1920-1955
Tiv 1931-1949
Tzeltal 1938-1965
Daly & Wilson (1990) Human Nature 1: 83-109
Most common situation…
Men killing men
Motives…
• Majority of all homicides (37%) are
“altercations of relatively trivial origin; insult,
curse, jostling etc” (Wolfgang, 1958)
• “Altercations appeared to be the primary
motivating forces… usually trivial, indicating
that many homicides are spontaneous acts
of passion” (Mulvihill, Tumin & Curtis, 1969)
Most common situation
continued…
Maladaptive By-product #1: Competition
–
–
–
–
Females are choosey
Look for dominant male
Insults focus on one’s dominance (weak, poor…)
Violence is an adaptive way of deterring/resolving
Examples…
• Movie
• Duels
– Men willing to die for honour
• ‘Chicken’
– Car race to edge of cliff
Most common situation
continued…
• Generally, violent behaviour is related to
fitness and lethality may be governed
by situation
– Less secure society, homicide may be seen
as ‘adaptive’ eg Dani Tribe, ‘kepu’
– Some groups, killing is a primary method
of survival eg Mafia
– Variance of income
Familicide
All homicides: Detroit 1972
Killing relatives: Detroit 1972
Relationship
Percent
Relationship
Percent
Unrelated
acquaintances
48%
Consanguinal
25%
Spouse
63%
Stranger
27%
In-laws
8%
Relatives
25%
Step children
4%
* 75% of all familial homicides were directed against
non-genetic kin
Uxoricide
Two possible theories
1. Risk increases with age
-Due to less value
2. Risk decreases with age
-Younger, more valuable mate increases jealousy/proprietary
Uxoricide continued…
The winner is…
• Risk decreases with age
140
Uxoricide Rate
120
100
80
60
40
20
0
15-19 20-24 25-29 30-34 35-39 40-44 45-49 50-54 55-59
60+
Wife's Age
Wilson & Daly (1998) in Rethinking Violence against Women (Dobash & Dobash, eds.)
Uxoricide continued…
Maladaptive By-product #2: Sexual
Proprietariness (Daly & Wilson)
• Males guard and control a valuable mate
• Males will change efforts due to…
– Factors affecting her mate value (youth, health)
– Factors affecting his mate value (status,
resources)
– Competition (sex ratio and qualities of rivals)
Uxoricide continued…
• Factors affecting his mate value &
competition
• Study by Dijkstra & Buunk (1998)
– Males are influenced by a rival’s dominance
– More dominant rivals evoke greater sense
of jealousy
– These types of responses may be adaptive,
but lethal consequences are not
Uxoricide continued…
Potential confound #1?
• Coincides with age of males that
murder more often…
– NO: Risk is elevated with older husbands
Age specific rates of killing same sex unrelated
persons
Uxoricide rates for wives aged 15-24 years
250
England & Wales
Homicides
per million
Uxoricide Rate
Canada
200
New South Wales
150
100
50
0
15-24
25-34
35-44
Age of Husband
Daly & Wilson (1999) Scientific American Presents 10(2): 8-14
> 44
Uxoricide continued…
• Does presence of step children increase uxoricide risk?
– YES: Evidence for greater shelter admissions and
uxoricides with presence of step-children (Daly & Wilson)
Shelter admissions (19861987) per 1000 per annum
8
Uxoricides (1974-1995)
160
6
120
4
80
2
40
0
0
Genetic
only
Mixed
Step
only
Genetic
only
Mixed
Step
only
Neonaticide, Infanticide & Filicide
• Who is at most risk?
1. Stepchildren:
Homicides per million
600
Stepparent
Genetic Parent
500
400
300
200
100
0
0-2
3-5
6-8
9-11 12-14 15-17
0-2
3-5
Child’s age (years)
6-8
9-11
12-14 15-17
Canada 1974 - 1990
Filicide continued…
Maladaptive by-product #3: Discriminative
Parental Solicitude
• *Remember, step-parental homicide is RARE,
however, presence of a step-parent increases
a child’s risk by over 100%
• Makes sense to invest more with children who
are genetically related
– Spend less $ per child on food
– Less financial support for college
Filicide continued…
•
Who is at most risk?
2. Younger children
Per capita rates of validated child abuse reports to
the American Humane Association, 1976
Stepparent
0-2
Wilson et al (1980) J Biosocial Science 12: 333 - 340
3-5
6-8
9-11
12-14 15-17
Filicide continued…
•
Who is at most risk?
3. Children with younger mothers
80
England &
Wales
1977-1990
60
40
20
0
< 20 20-24 25-29 30-34 >34
Age
60
Ayoreo:
Mid- 20th
Centruy
50
40
Homicides per Million Births
100
120
70
Homicides per Hundred Births
Homicides per Million Births
120
30
20
10
100
Canada:
1974-1990
80
60
40
20
0
0
< 20
20-24 25-29 30-34
Age
>34
< 20
20-24 25-29 30-34
Age
>34
Filicide continued…
Maladaptive by-product #4: Residual
reproductive value
- the older the mother the lower the
opportunity cost
Summary
• Definitions
• Conflict assay
• Adaptive vs Non-adaptive byproduct
– Male competition
– Sexual proprietariness
– Discriminative parental solicitude
– Residual reproductive value
• Homicide perpetrators/victims
– Homicide, uxoricide, filicide