Lecture 20: Aggression IV

What this lecture covers

This final lecture on aggression works backward through the causal chain introduced earlier in the course: from prenatal hormone exposure, to genes, to culture and ecology, and finally to evolutionary theory. It asks what each level actually explains about aggression and, just as importantly, what it explains about cooperation and peacemaking.

The lecture repeatedly overturns popular but flawed studies (androgenized girls, XYY males, the Kuwaiti incubator story) to show how easily correlational and confounded evidence gets misused, before ending on a personal account that frames why aggression resists tidy scientific explanation.

Key ideas

• Prenatal androgenization: masculinizes rodent behavior strongly, primates moderately, and humans only ambiguously; a famous claim that androgenized girls were 'more aggressive' turned out to just mean they wanted careers.
• Confounded human studies: androgenized girls and dizygotic twins with a male co-twin both carry confounds (reconstructive surgery, growing up with a brother) that make the hormone effect uninterpretable on its own.
• Simon Baron-Cohen's hyper-male hypothesis: proposes autism as an exaggeration of male-typical cognitive traits, tied to prenatal androgen exposure.
• Genes and aggression: single 'aggression genes' found in knockout mice often turn out to be genes for pain sensitivity, impulsivity, or general arousal instead; serotonin- and dopamine-pathway genes hold up but only within specific environments, especially early abuse.
• The XYY panic: a wave of 1960s-70s fear about the 'XYY male' after Richard Speck's murders collapsed once his karyotype was shown to be normal and no real link to violence was found.
• Pastoralist versus other cultures: nomadic pastoralist ('cultures of honor') societies show far higher rates of within- and between-group violence than agriculturalist or hunter-gatherer societies, illustrated by Nisbett's American South insult study.
• Pseudo-kinship and pseudo-speciation: cultures manipulate perceived relatedness to increase cooperation (military bonding, Bedouin peacemaking rituals) or to license violence by making an out-group seem less than human (wartime propaganda, the fabricated Kuwaiti incubator testimony).
• Conditions that select for cooperation: repeated (not a fixed number of) interactions, reputation, altruistic punishment, and the option to opt out of a game all favor cooperation over the 'tragedy of the commons.'

Walkthrough

Prenatal hormones and human ambiguity (0:04)

Building on organizational versus activational hormone effects from the sex lectures, animal studies show that prenatally androgenized females become clearly more aggressive and less maternal. In humans, congenital adrenal hyperplasia and DES-exposed girls were long claimed to be 'more aggressive,' a conclusion later shown to rest on the circular reasoning that wanting a career made a girl abnormal. Elevated IQ in these girls turned out to reflect parental IQ, not androgenization. Dizygotic twin studies show a small rough-and-tumble-play effect in girls with a male co-twin, but growing up with a brother is itself a confound. Simon Baron-Cohen's hyper-male hypothesis of autism is presented as one of the more productive uses of this literature.

Genes: real effects and false leads (13:11)

Genetically engineered 'aggression' mutations in mice frequently trace back to pain sensitivity or general impulsivity rather than aggression itself. Serotonin and dopamine pathway genes are the more solid candidates, but only predict violence in combination with early abuse. The Richard Speck case is used to show how the XYY panic spread through Congress and the military before collapsing when his karyotype was found to be normal.

Culture, ecology, and cultures of honor (23:21)

Nomadic pastoralist societies show consistently higher violence, standing armies, and warrior classes, linked to the vulnerability of mobile herds to raiding. Nisbett's staged-insult experiment found that student volunteers from the American South showed a much larger stress and testosterone response to a deliberate shoulder-bump insult than students from the North. Desert-dwelling pastoralist cultures also skew toward monotheism, while rainforest cultures skew polytheistic. A cross-country game-theory study found no cultural difference in willingness to punish cheaters, but a large difference in 'antisocial punishment' (punishing generosity), highest in Greece and parts of the Arabian Peninsula and linked to low social trust.

Pseudo-kinship, pseudo-speciation, and the amygdala (53:46)

Militaries and revolutions manufacture pseudo-kinship (band-of-brothers bonding, sibling address terms) to increase willingness to fight and die together; the same mechanism run in reverse, pseudo-speciation, makes an enemy seem less than human, illustrated by the fabricated 1990 Kuwaiti incubator testimony that helped authorize the Gulf War. Brain-imaging work by Elizabeth Phelps found amygdala activation on subliminal exposure to another race, but Susan Fiske's follow-up work showed this reverses when subjects are simply prompted to think of the person as an individual rather than a category member, and does not appear in people raised in diverse neighborhoods.

Evolutionary logic of aggression and cooperation (1:22:04)

Individual selection explains male-male violence over mates (illustrated by Napoleon Chagnon's controversial Yanomamo study) and male violence over denied sexual access. Kin selection explains cooperation among related chimp males and, per Daly and Wilson, higher abuse rates by stepfathers versus biological fathers. Reciprocal altruism requires repeated, unpredictable-length interaction, reputation, and the possibility of altruistic punishment to escape the 'tragedy of the commons.' The lecture closes with the WWI Christmas truce and the spontaneous 'tit-for-tat' non-aggression pacts that emerged in the trenches, and with Sapolsky's own account of standing over a murdered Ugandan soldier's body in 1979, offered as the reason aggression resists simple explanation.

Before you watch

• Watch Lecture 19 (Aggression III) immediately before this one; it sets up the serotonin, alcohol, and developmental material this lecture builds on.
• Review the organizational versus activational hormone distinction from the earlier sex-differences lectures.
• Recall reciprocal altruism and tit-for-tat game theory if covered earlier in the course; it is assumed knowledge here.

Check your understanding

1. Why does the 'more aggressive androgenized girls' finding from the 1960s-80s literature turn out to be flawed?
2. What happened to the XYY-male hypothesis of violence, and why?
3. What features distinguish nomadic pastoralist ('cultures of honor') societies from agriculturalist or hunter-gatherer societies in their rates of violence?
4. How did Susan Fiske's experiments change the interpretation of Elizabeth Phelps's amygdala findings on race?
5. What conditions does game theory identify as necessary for cooperation to emerge and be stable, rather than collapsing into a tragedy of the commons?