Lecture 17: Human Sexual Behavior III and Aggression I

What this lecture covers

This lecture closes Sapolsky's unit on the evolution of sexual behavior and opens the course's next major topic: aggression. The first half answers a specific question: given that natural selection favors passing on copies of your genes, why do traits like homosexuality persist, and what features make a potential mate attractive across species? The second half asks a different question: what actually counts as aggression, and why is the same violent behavior sometimes celebrated and sometimes condemned?

Following the course's evolution-to-biology arc, this lecture stays at the evolutionary and behavioral level. It sets up the neurobiology of aggression covered in the following lectures. After watching, you should be able to explain three competing evolutionary theories for homosexuality, describe several signals of attractiveness and why they evolved, and explain why the lecture treats "context" as the central variable in understanding aggression rather than the intensity of the behavior itself.

Key ideas

• Gender-dependent genetic argument: a trait that decreases reproductive success in one sex but increases it in the other can still be selected for if the benefit outweighs the cost; consistent with data that sisters of gay men have above-average reproductive rates.
• Helper-at-the-nest model: proposes that individuals not passing on their own genes directly instead boost their siblings' reproductive success, predicting increased reproductive rates in both siblings, which better matches the observed data than the gender-dependent model alone.
• Facial symmetry: an ancient and cross-species signal of health, since developmental or health problems tend to produce asymmetry; humans detect subtle asymmetries from infancy.
• The handicap principle (Amotz Zahavi): exaggerated secondary sexual characteristics work as attractiveness signals precisely because they are costly, so displaying them proves the animal has energy and health to spare.
• Homogamy: the strong cross-cultural tendency to mate with someone similar to yourself in religion, age, ethnicity and socioeconomic background, balancing the risks of inbreeding against the benefits of kin-related cooperation.
• Context over intensity: the lecture's central claim about aggression is that the biology of how to be violent is not what varies or goes wrong; what varies is whether the setting makes that violence acceptable or monstrous.
• Cross-species violence: competitive infanticide, weapon-making, organized "border patrol" killing, and behavior resembling genocide have all been documented in other primates, undermining the idea that lethal violence is uniquely human.

Walkthrough

Evolutionary theories of homosexuality (4:05)

The lecture opens by revisiting why homosexuality is a puzzle for evolutionary biology: if a trait reduces the average number of offspring, why hasn't it disappeared, especially given how widespread it is across cultures and species. Three theories are presented: the heterozygote vigor model (borrowed from the sickle-cell/malaria pattern, with little supporting evidence), the gender-dependent genetic model, and the helper-at-the-nest kin-selection model, which the data most strongly support.

Facial symmetry and composite faces (9:12)

Sapolsky traces the discovery that composite (averaged) faces are rated as more attractive back to 19th-century criminology experiments that accidentally produced highly symmetrical, attractive faces instead of "criminal" ones. He connects this to studies showing humans, other primates, and even two-month-old infants prefer symmetrical faces, and describes a study linking facial symmetry to dancing ability as an indirect effect of confidence rather than the dancing itself.

Secondary sexual characteristics and the handicap principle (17:23)

This section explains Zahavi's handicap principle: showy, costly traits (antlers, bright coloration) signal health because only a healthy animal can afford to waste resources on them. Evidence includes marsupial mice with more fertile sperm correlating with more exaggerated traits, and immune-system molecules generating the same pigments used in facial coloration. The lecture also covers a counterexample in vultures, where coloration can signal infection risk rather than immune strength, showing the theory can be argued both ways.

Confounds in mate-choice research (35:43)

Sapolsky highlights two problems that complicate this literature: self-fulfilling prophecies (mothers investing more in offspring from attractive fathers, making those offspring appear to have "better genes"), and bandwagon effects (a study on birds showing females become more receptive to a male after seeing him surrounded by apparently interested females).

Homogamy across cultures (41:49)

The lecture presents statistics showing more than 90% concordance in religion, age, race and socioeconomic background among married couples, with lower but still significant concordance for traits like IQ and height. It links this to earlier lectures on kin selection and inbreeding avoidance, citing Icelandic genealogical records showing peak fertility around third-to-fourth-cousin marriages, and a David Buss cross-cultural study finding that despite gendered differences in mate preferences, both sexes rank "someone who is nice to them" as the top trait.

Defining aggression across species (56:57)

The second half of the lecture pivots to aggression. Sapolsky opens with a personal anecdote about tripping an opponent during a soccer game to argue that the moral status of a violent act depends entirely on its social context, not on the act itself. He then dismantles the idea that humans are uniquely violent, citing chimpanzee infanticide, tool-based weapons, and coordinated "border patrol" killings documented by Jane Goodall, alongside evidence of reconciliation and proto-empathy in monkeys, chimps and even rats.

Before you watch

• This lecture assumes familiarity with the course's earlier discussions of sexual selection, male and female reproductive strategies, and kin selection, covered in the preceding lectures on sexual behavior.
• A basic grasp of natural selection and inclusive fitness helps make sense of the homosexuality and homogamy sections.

Check your understanding

1. Why does the helper-at-the-nest model fit the observed reproductive data better than the simple gender-dependent genetic model?
2. How does the handicap principle explain why costly, exaggerated traits can be honest signals of health, and how does the vulture example complicate that story?
3. What is homogamy, and why might moderate genetic relatedness (such as third or fourth cousins) produce higher fertility than either very close or very distant relatedness?
4. According to the lecture, why is context, rather than the intensity of a violent act, the key variable in understanding human aggression?
5. What evidence does the lecture use to argue that humans are not the only species capable of organized killing or empathy?