I watch “Human Behavioral Biology” course by Robert Sapolsky (from Stanford) on Youtube. It is very interesting. Here is my summary of the first three lectures.

Lecture 1: “Introduction to Human Behavioral Biology”

What the course is about

What is common between “having a period”, “having a brain tumor”, “eating a lot of junk food” and “taking anabolic steroids”? All have been successfully used in courts as a defense for a murderer.

Points of the course: 1) Sometimes what happens in your body can dramatically influence what happens in your brain (examples above). 2) Sometimes something in your head can affect your entire body (e.g. when one imagines that they will die their heart starts beating faster). In other words, the course is about the connection between physiology and behavior, emotions, memories.

Categorical thinking

We divide everything into categories, because this makes it easier to store information.

Illustration of spectrum divided into categories

E.g. it is much easier to remember whether you saw a triangle, a square or a circle than some weird shape, because we don’t have a word (i.e. a category) for latter.

Disadvantages of categorical thinking:

  1. You can’t differentiate two facts that fall in the same category (b & p in Finnish or two similar colors) - you underestimate their difference.
  2. We have trouble seeing how similar things are right across the category boundaries (e.g. 65% on the test leads to fail and 66% to pass) - we overestimate the difference.
  3. When you pay attention to boundaries, you miss the big picture (e.g. phone numbers are supposed to be 3 digits followed by 4).

Avoiding categorical thinking

The goal of the course is to think about biology of behavior without falling into categorical thinking. The category (bucket) you are in affects how you explain the world. We want to avoid this.

There are 3 main challenges - recognizing when we: 1) just like other animals and accepting that. E.g. ovulation synchronization in females living together. 2) we appear to be like any other organism, but do something very different. Examples:

  • chess master capturing a piece feeling like a baboon who won a fight
  • we get stressed when reading something bad happening to a character in a novel.
  • we can feel compassionate about other species. This is the same physiology as in other animals, but we use it in unrecognizable ways. 3) do something that no other animal even remotely does (e.g. non-reproductive sex, language use).

In the course we will overview various buckets first - evolution, molecular genetics. Then we will look at behaviors and consider them through each bucket separately.

Lecture 2: Behavioral Evolution I

Darwin didn’t discover evolution. He came up with a mechanism for it - natural selection. Darwinian evolution:

  1. there are heritable traits (i.e. genetic)
  2. there is variability among those traits (ways in which they can occur)
  3. some versions of those traits are more adaptive (work better for you) then others
  4. this is not about survival, but reproduction. When you have all of these, you get evolution and populations.

When you add mutations - you can get large changes.

This applies well to physiology, but how to apply this to behavior? Exactly the same:

  1. there are heritable behaviors (i.e. behaviors have genetic components, this is an important assumption)
  2. types/classes of behaviors have variation
  3. some are more adaptive than others.

There was a false belief that “animals behave for the good of the species”. Actually animals behave to pass as many copies of their genes as possible (reproduction of the fittest). Sometimes this may look like behavior for the good of the species.

Ways to pass your genes:

  1. Individual selection - maximize number of times you reproduce through natural selection (useful traits improving adaptability) and sexual selection (arbitrary traits).
  2. Kin selection - helping out relatives to reproduce (cooperate with relatives to a different degree, which depends on how closely they are related to you) by decreasing your own reproductive success.
  3. Reciprocal altruism - cooperation among non-relatives, but it must be reciprocal (you must get something out of it and not more than you put it), e.g. group hunting.

Prisoner Dilemma

Reciprocal altruism leads to cheating and being good at detecting cheating against you. When to cheat and when to cooperate is studied in game theory. The basic “model” is Prisoner Dilemma.

“Tit for Tat” strategy

  1. You cooperate
  2. Next time:
    • If they cooperated last time, you cooperate
    • Otherwise you cheat


  1. Nice (kind)
  2. Retaliates
  3. Forgiving
  4. Deterministic.

This drives other strategies into extinction when multiple agents play multiples games with each other.

However, there is a vulnerability - signal error. What if there is a possibility of a mistake (noise in the signal), e.g. cooperation looks like cheating. This wipes out 50% of cooperation:


Forgiving Tit for Tat (fTfT)

Improved version of TfT. Like normal TfT but after “saw pattern” for 5 rounds - forgive and cooperate. fTfT outcompetes TfT when you can have signal error. It has another vulnerability - it can be exploited by strategies without forgiveness.

“Tft -> fTfT”

Start off with pure TfT. If for N rounds the other party didn’t cheat, switch to fTfT. I.e. you begin to “trust” someone and this protects you from signal error.


When I do something and I get rewarded, do it again next time. If I lose, I switch. This allows you to exploit someone, who is forgiving.

Random mention: “The political uses of madness” - an article by Daniel Ellsburg on the optimal benefits of perceived madness (source1, source2).

Examples in nature

  • vampire bats. Females share their nest with non-relatives. Everyone feeds all babies. When they think that one female cheats on them, they stop feeding her kids.
  • black hamlet fish. They can change their gender. Pair of them flip their gender back and forth. Being female is more “expensive”. If one fish ends up being a male too many times, other fish will stop cooperating with them (TfT).


  • lion. Non-relatives in a pride. If you fake a sound of other lion in the bush, in many prides one lion stays behind the group, while all other go to check and risk by doing this. However, this lion is not punished. In real world you can have many ongoing games at the same time or others just don’t pay attention.
  • naked mole-rat. Big cooperative colonies (possibly non-relatives). 1-2 animals don’t do any work, but no one punishes them. They are even larger, because they eat more. However, when the rainy season comes they plug the entry to the tunnel with their bodies (example of another ongoing “game” and role diversification).

Application to behavior analysis

How to use the fact that species maximize copies of their genes to analyze behavior? Imagine that we have two skulls - one male and one female. Just by their relative size to each other we can conclude a lot:

Males much larger Males the same
male aggression high (their body is built for this and females select for this) low
variability in male reproductive success high (5% males do 95% of reproduction) low (every male gets 1-2 kids)
what females want large body size “good personality” - parental behavior (e.g. male birds bring food to females as evidence that they can feed kids)
parental behavior in males low (not interested in kids) high (care about kids)
life span high variation no difference by gender
twinning rare (impossible to feed on your own) more frequent (male helps to feed)
kids abandonment by females rare (kids will die for sure, male won’t take care) more frequent (male will feed them anyway)
species kind tournament pair bonding monogamous
example peacock (feathers show that you can waste a lot of energy) tamarin monkey (one can’t distinguish males and females visually)

We could see this only by ratio of male and female skull sizes. Humans fall in between these two kinds of species.

Lecture 3: Behavioral Evolution II

Let’s use “individual maximizes number of copies of their genes” to explain more behaviors.

Explaining infanticide

Babies are cute - they are trying to reduce aggression by this. There are species with infanticide (e.g. lions). Patterns:

  • adult male is the one killing
  • infants who are most likely to be offsprings of other males are killed
  • average interbirth interval in females is larger than the average tenure of high ranking male.

In other words, the new alpha male e.g. just killed the previous one, but all females won’t ovulate for 2.5 years, while you will be alpha male only for 1.5 years. Thus, by killing kids:

  • reproductive success of other males is decreased
  • females will start ovulating sooner (by stopping nursing).

This illustrates that individuals optimize number of copies of their genes instead of doing good for the species. Another example is mountain gorilla. They have infanticide even though they are at risk of extinction. In other words, they even endanger their species more to increase their own reproductive success.

Peculiarly infanticide does not happen if previous alpha was a relative. Two male lions can even share a pride of they are brothers (adelphic polyandry).

Females have developed number of adaptations due to this as well. In some species females have spontaneous abortion due to smell of the new male (there is no point to invest more resources into the kid, since it will be killed anyway). Females can also try to protect their kids. If they are older, they will risk their life more (they are less likely to have another kid). Maternal grandmothers defend even more (they won’t reproduce at all). Another strategy is to have pseudo estrous (i.e. fake ovulation) during pregnancy. The new male mates and thinks that the kid is his.

Baboons do blackmail kidnaping of kids. If alpha attacks an individual, they can grab alpha’s kid implying “attack me and I will kill the kid”.

Gender ratio fluctuations

In tournament species having a son is a volatile way (gamble) to pass your genes (may work out very well, but most likely won’t). Having a daughter is a conservative way (they will have few kids, but with high probability). However, giving birth to a male requires more energy. Higher ranking females have more sons (can take the risks). During environment distress (e.g. famine) more daughters are born. This can produce sex ratio fluctuations (having sons is better when there are few males).

Intrasexual competition

Males and females can have different reproductive goals.

This can be achieved by means of imprinted genes - their effect depends on which parent they come from. There is a genetic mechanism to achieve this. Examples of such genes and their usage in intrasexual competition:

  • Imprinted genes coming from father tend to improve fetal growth (i.e. the kid should get as much energy from the female as possible, males in migratory species do not care about future reproduction of a particular female, but only about their kid).
  • From mother - slow down fetal development (this particular kid is less important to a female than their overall reproductive life).

This mechanism malfunctions if one of the genes gets disabled due to a mutation.

There are imprinted genes, which work only after birth. E.g. from male one can get “suck more milk” (i.e. get more calories from female). You only see this in tournament species.

Humans have imprinted genes. By their quantity we are again in between pair bonding and tournament species.

There is sperm competition in polyandrous species. In some fly species sperm makes toxins that kill sperm of other males. However, they cause harm to females too. This makes no sense for the good of species, but does for individual success.

Direction of male aggression

There are species (e.g. chimpanzee) with exogamy (individuals leaving the group after puberty to avoid interbreading). If males leave the group - you get high aggression inside the group among males (they are not relatives). If females - among groups (chimpanzee genocide and border patrols). Human military tries to create this kinship artificially, chimpanzees have it by nature.


You can start cooperation by reintroducing highly interbread group (close relatives) into general population. This small group can outcompete others (due to their kinship) and others are forced to start cooperating as well. This is a push towards group selection (highly interbread group with kinship cooperation can drive others who don’t cooperate to extinction). In humans - trust in business or low interest loans.

It can be the case that strategy A dominates B when there is only one individual in each group, but A is dominated by B when there are many individuals in each group. E.g. a group of aggressive chickens would injure each other. However, when there are only two chickens, an aggressive one will outperform non-aggressive. This is a group selection as well (but due to personal benefit in the end).

Thus, we observe multilevel selection:

  • single gene level
  • individual
  • group


Criticism of applying Darwinian principles to behavior:

  1. Heritability of behavior - what are the molecular principles driving this?
  2. Adaptiveness - everything you see has an adaptive explanation for why it emerged evolutionarily - there are non-adaptive traits. Why do humans have chins? Otherwise you can’t get a flat face. These are so called spandrel traits, they are just an inevitable result of other traits.
  3. Gradualism - evolutionary change occurs in gradual incremental steps.

There is also political criticism - founding social biologists were white south American males (i.e. they could just be justifying male domination because this is positive for them).

Summary of following lectures