Shared publicly  - 
Human Sexual Behavior III & Aggression I. 100m Robert Sapolsky video.

Part 3 of 3 in a broad biological survey on the nature of human sexual behavior focuses on attractiveness. The second half of the video introduces part 1 of 4 in a broad biological survey of empathy and aggression. Extensive notes below.

17. Human Sexual Behavior III & Aggression I

Evolutionary theory suggests that the caloric expenditure of sperm (cheap) vs. eggs (expensive) implies more promiscuity on the part of males.

Female-female competition. In some pair-bonding species (such as new world monkeys and many bird species) females compete for the better "parental" males and so they are often more aggressive, have larger body size, and more prominent secondary sexual characteristics (those traits that appear in puberty).

5-20% of the population is homosexual in every human culture ever looked at. The evidence for a genetic basis for homosexuality is not established, so there may not be any role for natural selection. There are three theories for the selective advantage or neutrality of homosexuality. Hetereozygotic vigor argument (like sickle cell anemia where the homozygotic version is a disease but the heterozygotic variation protects against malaria): so it could be that homosexuality is the non-selective form but in heterozygotes it is adaptive (very little evidence). Gender-dependent argument: in one gender the genetically influenced trait is adaptive while in the other gender the trait is maladaptive and leads to homosexuality (evidence: the sisters of gay men tend to be more reproductively successful). Helper at the nest kin selection model: the individuals who are not passing on copies of their genes are helping their siblings (so both sisters & brothers would have higher reproductive success and studies seem to mostly support this model).

Traits that are attractive.

Facial symmetry. Francis Galton developed composite photography which he used to determine the face of crime. Sapolsky belittles the technique and asserts that such composites average to a more attractive face (corroboration: The symmetry seems to be the quality that makes such composites attractive. The literature suggests that symmetry is a good indicator of health. babies at two months already prefer more symmetrical faces (rats & nonhuman primates also show a preference for symmetry). A study in Nature showed that people with more symmetrical faces were better dancers (this study was retracted by Nature in Dec 2013: The faces of women when ovulating are slightly more symmetrical (

Secondary sexual characteristics. Zahavi's handicap principle: the bigger more garish secondary sexual characteristics is a signal to the world of health, good immunity, or fertility (a signal that one has so much energy to conspicuously consume resources to build this garish trait to attract a mate). Sapolsky claims there is lots of evidence for the principle. Marsupial mice: those individuals with the more exaggerated secondary sexual characteristic had more fertile sperm. Immune system molecules may create the more showy trait. A study of women from many industrialized societies correlates inversely between a preference for men with big jutting jaws, high forehead, and musculature and life expectancy and economic health of that culture. Organisms do not want disease, so they want evidence of a strong immune system. They are good at detecting the smell of parasitic or other infections and avoiding those individuals. Some individuals may cheat: to uncouple their health from their secondary sexual characteristics (some bird species will put in extra energy to enhance their traits when they are sick: reproduce now or never?). Of course, counterstrategies and another coevolutionary back and forth is seen.

A problem with the handicap principle: "there is a certain way to frame things so that you never ever can get a finding that disagrees with your general stance". Vultures have sexually dimorphic faces. Male vultures faces are orange due to carotenoids from consuming ungulate feces with parasites. So dramatic secondary sexual characteristics can both be a marker of health or healthy enough despite some disease. The handicap principle remains controversial. Lions are a tournament species with strong sexual dimorphism (manes): a black mane would be most attractive based on the handicap principle, but it would heat your head in the African sun. So there are complex balancing issues.

Some secondary sexual characteristics are markers of fertility, such as, external swellings in ovulating females (estrus). Baboon males prefer females with larger swellings. Swellings are a marker of estrogen levels, greater fertility, and greater health. The biggest swellings may incur an extra 25 pounds of water weight and yet I can still run around the jungle as a marker of fitness and a source of attractiveness. Humans are concealed ovulators. Human females have no swellings, but a larger hip to waist ratio implies more fertility (child-bearing pelvises, development health augurs well for better fertility). Human males across cultures find larger hip to waist ratios more attractive. This could be a Western bias: the minimal literature on hip to waist ratio among non-Western cultures at first contact suggest that the attractiveness of the trait may be less strong. It is a widespread trait in almost every culture studied. Human females find big jutting jaws, high forehead, and musculature more attractive and these are markers for high testosterone levels during adolescence. Surprisingly, women rate rounder faces as more likable, more honest, more trustworthy, but less desirable! There are subtle differences that people are only subliminally aware of. During ovulation, women's attractiveness for these male secondary sexual characteristics increases (as does pheromone sensitivity).

Confounds: female birds who mate with more attractive males, invest more effort in their kids who have bigger egg size, and therefore they do survive better. So is this a self-fulfilling prophesy or is it the handicap principle? Lee Dugatkin at University of Louisville (I could not find this study, could be a spurious citation): in situations where the female rejects the male, he makes the male appear to be very popular (stuffed females around the rejected male) then the female who spurned him is more likely to initiate proceptive behavior. This bandwagon effect has been shown in many species, but it may be a social contagion not a sign of fitness nor the handicap principle.

In many pair-bonded species attractiveness is shown by demonstrating parental competence (bringing worms) not markers of health or fertility. In most non-human primate species studied, females are most attractive to males (independent of estrus swelling) when they have already had a few kids which might be another marker of parental competence.

In many species and in humans, individuals tend to be attracted to others who are "just like me": homogamy (mating with someone who is homogeneous or similar in traits). In the US, greater than 90% chance that couples share the same religion, are within three years of each other, have the same ethnicity, race, the socioeconomic status (SES) of their childhood, political views; greater than 40% concordance for having similar IQ, level of education; 20-40% concordance having the same percentile for height for their sex, weight, hair color, lung capacity, width of nostrils or eyes. This is probably driven by kin selection: optimal fertility is roughly 3rd or 4th cousin. Homogeneity signals that we might be distant relatives. Studies of hunter-gatherers shows they tend to mate with people who grew up less than 40 kilometers away. In traditional agricultural societies, mates tend to grow up less than 10 kilometers away. Homogamy is widespread. In a study of 200 years of Icelandic marriages, optimal survival of offspring are for 3rd & 4th cousin marriages. In the US, younger people are more likely to make less homogamous mate choices. Perhaps as we get older we become more closed-minded? For religion, there is a secondary increase in heterogamy between 50-60. Could that be due to lack of choices, because children are now not a factor, a midlife crisis, or they waited for their parents to die?

David Buss published a wide-ranging cross-cultural study which found that in all cultures women want mates who are older than them, men looking for women younger than them, women citing a man's economic prowess as important, men having a preference for markers for fertility, both sexes rated as their highest preference for someone who is nice to them.


Sapolsky's soccer aggressiveness made him feel wonderful.

Aggression is about knowing when to be aggressive: finding the appropriate social context. We love violence, we get excited by it, and will pay good money to see it, and we'll even gladly join in when it is the right kind of violence. The same behavior can get you all kinds of awards and "differential reproductive success" and the exact same actions with your muscles could be among the worst things that one human could do to another. Context is critical.

What about violence and aggression is unique to humans?

Sarah Hrdy found competitive infanticide in langur monkeys upending the dogma that no other species besides humans kills its own. By now there are 20-25 species that have competitive infanticide. Humans are not the only species that kills its own kind. Jane Goodall with Chimps showed that humans are not the only ones who kill in a premeditated, strategic, Machiavelian kind of a way: females killing each other's babies, males killing males. Now we know that Chimps also make weapons for killing. Chimps are female exogamous, so the males in a group are related. Goodall observed "border patrols": the males of a group buildup a high level of excitation and then patrol the boundary of their territory and kill any males from other groups they encounter. She also documented examples of the males in one group systematically killing all of the males of a neighboring group: genocide (killing all individuals based on what group they belong to).

Reconciliation (increased likelihood of affiliative behavior after an aggresive encounter) was once thought to be uniquely human, but there are now reports of reconciliative behavior in a couple of dozen other species. This was first reported by Frans de Waal in Rhesus monkeys. It has been noted in dolphins, whales, and gorillas. Marina Cords showed the odds of reconciliation increase when it is a more important relationship: macaques show more reconciliation if they have a history of cooperating to get food. Instead of the game theory framing, she prefers to frame it as supporting a more valuable relationship. In baboons, females will reconcile but not males. Bonobos have aggression, but it is usually followed by reconciliative behavior afterwards.

Humans are not the only species with a sense of justice. Franz de Waal showed that chimps that establish a cooperative relationship for getting food, will share the food even if only one of them controls the food gathered by the cooperative effort.

Empathy is not unique to humans. In chimps, Frans de Waal found unjustified pommeling by high ranking males elicits more grooming by females in the troupe (but not if the low-ranking guy was provocative). A group from McGill in a study in Science showed that rats hearing ultrasonic alarm calls of their cagemates are more sensitive to pain than agitated rats from out of the group. Humans are the only species that can be moved by suffering on the other side of the planet or artworks or fictional characters in books or movies or even by neglected "child" lamps. Humans have "truth and reconciliation" commissions in South Africa, The Balkans, and Rwanda, sometimes there is even forgiveness. "I will let no man spoil my soul by causing me to hate him": an unprecedented psychology. Helen Prejean, a Catholic nun who ministers to the needs of death row inmates in maximum security prisons whose biography "Dead Man Walking" has been made into a feature film, opera, and a play, says "the less forgiveable the act, the more it must be forgiven; the less lovable the person, the more they must be loved": a distinctly human take on empathy with a logic no one in the animal world could understand.

Humans are less territorial and have less distinct dominance hierarchies. Top/down (despotic) hierarchies have a single aggressive individual who enforces unequal distribution of resources based on violence or threats of violence (baboons, chimps, rhesus monkeys). Bottom/up (egalitarian) hierarchy the top individual is only there via the cooperation of everyone else in the troupe (vervet monkey).

Patas monkey ethology finds almost no male-male violence, but put two males in the same cage and they will fight to the death. In the wild the social structure keeps males as far apart as possible: social cues prevent aggression from happening. Are they a violent species or not? Animal "violence" can also mean getting dinner (predation). Is it violent to eat meat? Or to kill your dinner? What constitutes violent behavior?

Rough and tumble play is ubiquitous in the animal world. Play is one of the last behaviors that goes away during famine: it is deeply hardwired. Primate studies suggest that aggressive play is not for practice, it is establishing the dominance hierarchy that will emerge later.

Unique human forms of aggression: although we can be as violent as a chimp cudgeling one another, we can be violent by doing nothing more taxing than pulling a trigger, looking the other way, releasing a bomb from 30,000 feet, passive aggressive, damning with faint praise. Examples. Two young kids have a tussle and one breaks the other's easter egg. Teacher says paint her another, so she paints a black one "here's your stupid egg". The other bursts into tears upon seeing the egg. Cooperating with the letter of the law while doing as much violence as possible to the spirit of the law. Sapolsky's wife got cutoff in traffic and responded "I'm going after this guy" and when he is trapped at a red light goes to his open window and say "anybody who could do something like that needs one of these" and flings a lollypop at him. At Nellis Air Force Base just outside Las Vegas in an air conditioned room people control drones killing people 12,000 miles away and then go home to their wife and kids: another day at the office.

Humans are the only species that can psychopathologically confuse sexual behavior with aggressive behavior (sadism & masochism): that appears to be a unique trait.

Humans are not the only species with empathy, but we have it in unique realms.

Human empathy and aggression are very complicated behaviors.

As with sexual behavior, the limbic system is important for understanding the biology of empathy and aggression.

The amygdala with its role in fear and anxiety is important in understanding aggressive behavior. Sapolsky highlights the connection between fear and aggression in the amygdala. Animal studies show the amygdala is involved in aggression (destroy it and no more aggression). In the 1960s & 70s there were thousands of court ordered amygdalectomies (surgical lesion to destroy the amygdala on both sides of the brain) to decrease aggression (it worked, but "there would not be a whole lot of a person left afterward"). In rats, stimulation experiments produce aggressive behavior. A rare form of epilepsy where the focus of the seizure is in the amygdala (many epileptic seizures are preceded by stimulation in the region of focus: smell, sound, math equation, etc.) has seizures preceded by a sudden furiousness. So uncontrolled stimulation in the amygdala results in aggression.

Charles Whitman in 1966 climbed the famed clock tower at the University of Texas at Austin and shot 32 people before killing himself. He had a tumor in his amygdala. Brain scan studies: show something to make someone angry and note that the matabolic rate in the amygdala increases. The amygdala is larger in people with post traumatic stress disorder. People with amygdaloid lesions have difficulty detecting faces with angry emotions, they are more trusting of others, and more likely to forgive. Antonio Demasio eye tracking study: people with amygdaloid lesions make less eye contact. The amygdala directs you to look for potential fears in the world. Testosterone in males makes them look harder for scary things and lowers the threshold for response.
writerconsidered's profile photoCJ Fearnley's profile photo
I do not have time to write transcripts, so my notes are a quick and dirty blend of quotes and interpretations. They are not edited and reviewed and probably include errors. But you do get a summary of the contents of the lecture. Watch the video itself to double check any facts.
Add a comment...