Human Sexual Behavior - part 3 & Aggression - part 1.

There is female-female competition in pair bonding species, since male takes care of the kids (if he believes that he is the father). A female wants a male, who is good at this.

There are species (e.g. New World monkeys) where females are more aggressive than males and have larger bodies. I.e. exactly opposite to the majority of species.

Evolution of homosexuality

The estimated homosexuality rate is from 5% to 20% (across all human cultures).

There are 3 theories (all based on the assumption of genetic component of sexual orientation):

  1. heterozygotic-vigor argument

  2. Gender dependent genetic argument - sisters of gay men have higher than average reproductive rate

  3. Helper at the nest - homosexual individuals spend resources on helping their siblings and, thus, pass their genes indirectly. In other words both sisters and brothers of homosexual siblings should have increased reproductive rates (this has been observed).

Looking good

Facial symmetry

In 19th century there was a theory of “criminal face”. They overlapped faces of criminals and produced a composite face. Everyone perceived this face as very good looking. When you average faces, they look more attractive than the individuals they came from. The more faces there are in the composite, the more attractive it is perceived.

The explanation - you get highly symmetrical face. Symmetry is a good reliable marker of health. Wildly asymmetrical faces are typically results of developmental issues.

People can pick up astonishingly subtle asymmetries in faces. Two months old babies already prefer to look at pictures of more symmetrical faces. This is also observed in other species.

There was a study, which has shown that people with more symmetrical faces are better dancers. They recorded people dancing and removed all features of a person (e.g. replaced with a 3D model). Other people rated the resulting dances. Dances of people with more symmetrical faces (the faces were not shown to the raters) were rated higher. Possible explanation: people with more symmetrical faces are more attractive, thus, treated better, thus, more confident, thus, more comfortable dancing.

Women faces at ovulation become a bit more symmetrical.

Secondary sexual characteristics.

There is a handicap principle from Zahavi. Individuals show through secondary sexual characteristics that they have so much energy that they can afford to spend it on such not immediately useful features. Such features are also often markers of health and immune system (lots of evidence).

In Marsupial mice - males with more dramatic secondary characteristics have more fertile sperm.

Study - the better a western country is doing (life expectancy, economy, quality of life), the less bias women have for secondary sexual characteristics in males (big jaw, high forehead, better developed muscles).

One can also detect whether other individual is infectious via secondary sexual characteristics. Many such characteristics advertise having no communicable diseases. Many species are good at detecting smells of parasitic infections.

There is also cheating - a selection to uncouple these characteristics from one’s health. In birds, one observes sick animals spending too many calories on developing their secondary sexual characteristics.

Problem with all such research - intensity of such characteristics has to reflect quality of immune system. Facial coloration linearly depends on immune system. However, in vultures - one gets carotenoids for face coloration by eating ungulate feces. They also contain a lot of parasites. This way they say “see, I can handle so many infections, thus, my DNA must be really good”. This is a controversy.

Lions are highly tournamental species. Black mane in males is the most attractive and the most expensive to develop. However, your head heats up more, so you have to balance the attractiveness of your mane with spending more energy on your thermo regulation.

Some such characteristics are direct markers of fertility. In primates females have external swellings when ovulating. Humans are concealed ovulators. Some baboon females have bigger swellings than other. Male baboons prefer larger swellings. They are a marker of estrogen levels. Females with larger swellings have better infant survival rate. However, it is also a handicap, one can gain 25% more weight due to water retention for the swelling.

In humans there is waist-hip ratio. The bigger this ratio is, the better the fertility is. However, in more traditional societies you don’t see this preference that much. Perhaps this is just a bias of western culture.

In human males - larger jaws & high foreheads, more developed muscle mass - markers of testosterone levels.

Rounder faces of men are perceived as more likable, more honest, more trustworthy, but less desirable.

When women ovulate, they prefer more expressed secondary sexual characteristics.

However, there is another problem with this line of reasoning. Let’s say a female gazelle mates with a male with amazing secondary sexual characteristics and gets a kid. She will expand more calories to take care of the kid, because it has great reproductive potential. In birds

  • larger eggs when mating with more attractive males. In other words, a female makes this a self fulfilling prophecy.

Also females take into account how other females perceive males. Lee Gugatkin did a study looking at female rejecting males. After a rejection, fake female birds were placed around the male to make him look really popular. The rejecting female was than more likely to do solicitive courting. In other words she is jumping in a band wagon: “If I don’t find him attractive, but others do, these traits must be actually good and I want them in my kids”.

In pair bonding species males show that they can be a good parent. Thus, there are markers of parenting skills. In non-pair bonding primates, females with kids are more preferred, since this is a marker of parental competence.


Individuals like other individuals, who are more similar to them. E.g. in the US for marriages there is 90% likelihood to share same religion, 3 year age window, race, ethnicity, socioeconomic status during childhood, political views. 40% for IQ and education. 20-40% - same percentile of height (taken per gender), weight, hair color, lung capacity, width of nostrils, width of eyes (probably markers for race and ethnicity).

This brings us again to kin selection. Mating with third to fourth cousin is optimal and also improves fertility. I.e. you should share great-great-grandparents (grand parents of your grandparents) or great-great-great-grand parents.

In agricultures villages people in marriages originally lived less than 10 kilometers away. In Iceland people are not more than 6th cousin apart and they have clear records going back centuries.

Children have optimal survival rate in third-fourth cousin marriages.

In the US people make less homogenous choices when they are younger. This illustrates that people get more close minded as they get older. There is an exception for religion, there is a second peak of likelihood to enter non-homogenous marriage around 50-60 years old. Such people often have already been living for a long time together. One possible explanation - they wait for their parents to die, in order not to disappoint them with such a marriage.

Commonalities across cultures

In every single cultures women are more likely than men to want older males. Men - younger females. Women look at economic prowess, men - fertility markers. This is researched by David Buss.

These are also cliches. Because in all cultures across sexes there is the same top ranked preference to wind up with someone who is nice to you.


You learn when to be aggressive, e.g. brutally tackling an aggressive football player is perceived much better than doing the same to Nelson Mandela. We welcome violence when it is of the right kind.

Aspects which were considered unique to humans, but are not:

  • Killing for pleasure. There is competitive infanticide. First observed in langur monkeys by Sara Hardy. Initially people explained this as a psychopathology and caused by humans invading their habitat. Jane Goodall observed killings in chimpanzees. They even make weapons and have organized violence. All adult males in the group are relatives, so they have cooperative aggression and border patrols. They get extremely excited and start patrolling their territory. When encountering a male from a different group, they will kill him. There are cases of male chimps systematically killing males of neighboring groups, i.e. genocide (killing because of the group one belongs to).

// While looking up this topic, I encountered “Gombe Chimpanzee War” article in Wikipedia with casualties and commanders. I was surprised. Looks like this was observed by Goodall.

  • reconciliation - doing something affiliative after aggression. Frans de Waal studies this. Observed in rhesus monkeys, dolphins, whales, gorillas - social grooming after fight.

Odds of reconciliation increase when the relationship is more important (e.g. in macaque). E.g. pairs which had history of cooperation (like bringing food together to a cage).

There can be gender differences - e.g. in baboons only females reconcile. In bonobos they have sex instead of grooming. One also requires aggression for reconciliation.

  • empathy - chimps, lab rats (studied by Frans de Waal). If a male is attacked by a more dominant male - an innocent bystander (i.e. not picking a fight) is far more likely to be groomed by females afterwards than the one who picked up the fight with more dominant male. Rats send ultrasonic alarm calls when restrained. Rats who hear the alarm have lower pain threshold, but only when the rat producing the alarm is e.g. their cage mate.

  • sense of justice. Chimps - after a cooperation in which one chimp gets the food, that chimp is more likely to share the food.

  • dominance hierarchies. Top down - single (despotic) most aggressive individual with uneven resource distribution due to threats, e.g. baboons, chimps, rhesus monkeys. Bottom up (egalitarian) - (e.g. velvet monkeys) - number one cooperates with everyone, if they get abusive - they get overthrown.

  • rough-and-tumble play. In famine times this is the last of behaviors
  • to disappear in kids.

Unique aspects:

  • sado- & masochism (due to confusing sex behavior with aggression).

  • unusual domains of empathy. E.g. an Ikea ad, showing a lamp being thrown out causing empathy reaction to the lamp and then “What’s wrong with you, it is just a lamp, a new one also works better”.

  • being passive aggressive, killing indirectly (e.g. by releasing a bomb or looking away). E.g.

    • a 5 year old child broke other kid easter egg and then was forced by paint a new one. They painted it completely black (complied with the requirement, but still were aggressive).

    • Professor’s wife. Once she was driving and they were cut off by someone. His wife started following that person and eventually reached them on a long red light. She came to the window and threw a lollipop inside with “Anyone who does something like that, needs this”.

    • In Nevada, many men every day wake up, kiss their family and go to work to control drone airplanes in Iraq killing people thousands of miles away.

  • Extreme empathy

    • truth and reconciliation commissions in South Africa.

    • “I will let no man to spoil my soul by causing me to hate him.”

    • Sister Helen Prejean: “the less forgivable the act is, the more it must be forgiven”.

Tricky aspects of studying aggression

Take Paras monkeys - no male fights in nature. Put two in a cage - they will fight to death. Their social structure is built around keeping males as far as possible from each other (the tension is high), thus, they live in very dispersed patterns. Are they non- aggressive (no fights) or extremely aggressive?

Ripping other individual to shreds can be an aggression or getting dinner.

Mechanisms in the brain

One second before a violent act in the brain - limbic system plays important role. E.g. amygdala. Individuals with destroyed amygdala are incapable to elicit aggression. In humans there were court ordered amygdalectomies in 60-70s. They would decrease aggression and a lot of other stuff. Basically there wouldn’t be a whole lot of a person left afterwards.

There is also stimulation evidence - produces wildly aggressive behavior. There is rare epilepsy in amygdala. Normally epileptics observe some signs of coming seizure - can be odor, seeing math equations, auditory. When amygdala is affected - people become extremely furious. Charles Whitman - mass murdered in 1962, was found to have a tumor in his amydgala. In 1970s there was Baader Meinhof gang in Germany, one of them had a tumor in her amygdala.

One’s amygdala activates when you evoke anger in someone. It also gets bigger in PSTD victims, which leads to increased frequency of violent behavior.

People with lesions in this area are very bad at detecting faces, which express anger. They are more trusting than average and more likely to forgive. There is a study by Antonio Damasio showing that such people don’t look at eyes of other people. In other words amygdala is able to direct you to look for aggression.