1. Conflict Between Groups
The long history of warfare and intergroup conflict.
Selection pressures and psychological implications:
Potential threat to survival.
Potential threat to mate choice.
(Implications especially for women.)
Potential opportunity to enhance mate value.
(Implications especially for men.)
- The male warrior hypothesis.

2. Buss, Chapter 10 (“Aggression and Warfare”):
Evolutionary perspectives on aggression.
Evolutionary perspectives on war.
Evolutionary perspectives on homicide.

3. Two different questions about human evolution and war:
“How does our evolved psychology help us to understand war?”
“How does war help us to understand our evolved psychology?”

4. “How does our evolved psychology help us to understand war?”
In other words:
“War is a mysterious puzzle! How can we use the principles of evolutionary psychology to explain the origins of intergroup conflict? What functionally-specialized psychological adaptations might account for humans’ long history of engaging in intergroup conflict?”
Selection pressures in ancestral populations
Psychological adaptations of one sort or another.
War and other forms of intergroup violence

5. “How does war help us to understand our evolved psychology?”
In other words:
“War is already easily explained! (It’s usually about competition over resources.) Now, how can we use the fact of intergroup conflict and the principles of evolutionary psychology to arrive at novel insights about human psychology? What functionally-specialized psychological adaptations might have evolved as a result of humans’ long history of engaging in intergroup conflict?”
Selection pressures associated with intergroup conflict in ancestral populations
Psychological adaptations resulting from those specific selection pressures
Implications and outcomes of various kinds (including, but not limited to, war)

6. Three specific selection pressures with psychological implications:
- Potential threat to survival.
- Potential threat to mate choice.
(Implications especially for women)
- Potential opportunity to enhance mate value.
(Implications especially for men)

7. Brief review: Outgroup members posed a threat to survival:
(Especially outgroup men)
(Especially when people are vulnerable)
Implications:
- Hard to unlearn fearful responses to outgroup faces.
- Stereotypic beliefs that outgroups are dangerous.
Additional implications:
- Especially hard to unlearn fearful responses to outgroup male faces.
- Stereotypic beliefs that outgroup men are especially dangerous.
More additional implications:
- “Dangerous” stereotypes are exaggerated when people feel more vulnerable.

8. Stereotypic belief that outgroup is dangerous
Further implications for intergroup conflict in the here and now:
“Fight” or “flight” responses to perceived threats.
Potential for behavioral decisions that create (rather than avoid) intergroup conflicts, and that perpetuate (rather than resolve) existing conflicts.
More aggressive and less conciliatory responses within context of inter-group conflict
Stereotypic belief that outgroup is dangerous
Vulnerability

9. Stereotypic belief that outgroup is less dangerous
Further implications for intergroup conflict in the here and now:
“Fight” or “flight” responses to perceived threats.
Potential for behavioral decisions that create (rather than avoid) intergroup conflicts, and that perpetuate (rather than resolve) existing conflicts.
Less aggressive and more conciliatory responses within context of inter-group conflict
Stereotypic belief that outgroup is less dangerous
Less Vulnerability

10. Intergroup conflict and threat to mate choice:
Underlying evolutionary logic:
- Rape and sexual coercion within the context of intergroup conflict.
- Threat to female control over mate choice.
- Potential costs to female reproductive fitness.
- Especially when women were most likely to conceive.
One implication:
- Menstrual cycle variation in female attitudes toward outgroups.

11. Also: This effect showed up most strongly among women who felt most vulnerable to sexual coercion, and who perceived outgroup men to be especially physically aggressive.
(Navarrete et al., 2009)

12. Intergroup conflict and opportunity to enhance mate value:
Underlying evolutionary logic:
- Choosy females and show-off-y males.
- Intergroup conflict provided a domain within which males could potentially “show off” and therefore gain greater access to choosy mates.
Some relevant evidence:
- Anthropological research on Amazonian tribal societies:
Male warriors had higher status and greater access to sexual partners.
- Archival data on U.S. World War II veterans:
Compared to other veterans, “war heroes” produced more offspring.
- Experimental evidence on perceived sexual attractiveness:
Women rated male “war heroes” to be especially sexually attractive.

13. British women presented with information about a male British soldier.
Three experimental conditions:
No War: “John has been in the military for 5 years now. He is the leader of a unit consisting of 4 men. John and his unit have not yet been to a war zone during their employment in the military. They have stayed in the UK and are responsible for the coordination of military personnel and equipment.”
War: “John has been in the military for 5 years now. He is the leader of a unit consisting of 4 men. John and his unit have been to Iraq recently. When they were there, their main job was the coordination of military personnel and equipment but they have also fought in a number of battles. After 6 months, John and his unit have returned home safely.”
War Hero: “John has been in the military for 5 years now. He is the leader of a unit consisting of 4 men. John and his unit have been to Iraq recently. When they were there their main job was the coordination of military personnel and equipment but they have also fought in a number of battles. After 6 months, John and his unit returned home safely. John was awarded a medal for individual bravery upon his return from Iraq.”
(Rusch, Leunissen, & Van Vugt, 2015)

14. Results: Perceived sexual attractiveness
(Rusch, Leunissen, & Van Vugt, 2015)

15. “Male warrior hypothesis”
If intergroup conflict provided males with opportunities for the attainment of fitness benefits, then there may be evolved psychological mechanisms that govern male attitudes and actions pertaining to intergroup conflict.
Some specific implications…

16. Sex differences in attitudes toward intergroup conflict.
Because men potentially benefited more from intergroup conflict, men are more likely to have positive attitudes toward intergroup conflict.
Attitudes regarding real-life conflict situations:
Support for military inventions: Men > Women
Support for continuation of ongoing wars: Men > Women
Behavior in experiments that simulate war games:
Unprovoked aggressive attacks: Men > Women

17. Just a reflection of traditional gender roles?
Or does it reflect something deeper?
Prenatal environment
Development of nervous system
Attitudes and behaviors later in life

18. Just a reflection of traditional gender roles?
Or does it reflect something deeper?
Prenatal exposure to testosterone
Development of a “masculinized” nervous system
Prototypically “male” attitudes and behaviors later in life

19. Just a reflection of traditional gender roles?
Or does it reflect something deeper?
Prenatal exposure to testosterone
Development of a “masculinized” nervous system
More aggressive attitudes and behaviors later in life
[for example]

20. Research on finger length ratio (“2D:4D”):
Indicates less prenatal
exposure to testosterone.
(More common among women)
Indicates more prenatal
exposure to testosterone.
(More common among men)
Behavior in experiments that simulate war games:
Smaller 2D:4D ratio associated with more unprovoked aggressive attacks.
(Among both men and women.)
(McIntyre et al., 2007)

21. Physical prowess and male attitudes toward intergroup conflict.
Male attitudes toward conflict are calibrated to the likelihood of actually obtaining the reproductive benefits associated with conflict.
Physically stronger men more strongly endorse the use of military force to resolve international conflicts.
(Sell, Tooby, & Cosmides, 2009)

22. Intergroup conflict and within-group cooperation.
The benefits of intergroup conflict (to men) were most likely to accrue if the ingroup was victorious; and a victorious outcome would have been most likely under conditions of within-group cooperation.
Implication: Intergroup competition  within-group cooperation (among men)

23. (Van Vugt, De Cremer, & Janssen, 2007)