Lecture 8, recognizing relatives.

Follow-up to previous lecture


The fact that humans have 5 fingers is an inherited trait. However, the variability of #fingers is exclusively due to environment, thus, heritability is 0%. People often assume that heritability tells whether a trait is inherited.

Previous disciplines

The previous three approaches (behavioral evolution, molecular genetics, behavioral genetics) are just descriptions at different levels. For example one can view epigenetics as either of:

  • the way how culture and environment affects biology
  • the way in which environment turns genes on/off
  • regulation of chromatin, remodeling, regulation of genes.

Behavioral genetics

  • classical - adoption studies, good at showing that environment has less impact than assumed.
  • modern - we suspect that a given gene has some behavioral effect, let’s see which. Alternatively, we suspect that this behavior is genetic - let’s see the link.

Observed effect size

When evaluating studies, keep the observed difference size in mind. For example, there was a study of 250k 18 years old in Norway, which has shown that first borns have higher IQ than second borns. There could be multiple ways to explain this:

  • Parental investment - no, they controlled for this, if you are the only child, you have lower IQ than first born with younger siblings.
  • “Tutoring” phenomenon
  • Mother age (egg quality) - they controlled for this too.
  • Each pregnancy causes immune suppression, a mother could become less healthy after the first pregnancy.

Up to age 12 second borns have higher IQ than first borns. By age 18 it gets other way around. A second born IQ reverts to the level of first born, if the first born dies.

However, the difference was only 2.3 IQ points (as Sapolsky says “sneeze at the IQ test and you get this difference”), i.e. not important at all (even though the study has been done in a very elaborate fashion).

Lecture 8: Recognizing relatives

Ability to recognize relatives can be present even in very primitive organisms. E.g. deer mice have 2 strains - monogamous and polygamous, the later has sperm competition. When sperm forms a clump, it travels better. However, in polygamous strain sperm tries to form a clump only with its own kind. In the monogamous strain, this is not the case (happily forming a clump with any sperm). In the polygamous case, sperm “knows” who it is related. This is example of single cells recognizing relatives.

You can show innate relative recognition by cross fostering.



Pheromonal communication can be used for relative recognition. One needs qualitative differences e.g. in urine (reflecting genetic makeup) and some mechanism to process the differences. This is achieved by creating a completely unique protein (major histocompatibility complex), which functions like a fingerprint. This protein is attached to each of your cells (at the surface). Your immune system uses it to detect your own cells. This protein can become soluble (saliva, urine) and give unique signature to pheromones coming off of you. These protein structure is also more similar in closer relatives.

Oxytocin and vasoprasin tune cells in olfactory system for better accuracy to recognize relatives and, if the match is good, take care of them (e.g. baby after birth).

There are only 2 areas in the brain, which have neurogenesis (new neurons generation) in adults - hippocampus (learning) and behind olfactory bulb. In rats, new neurons in the olfactory bulb are created before giving birth. This way the mother can detect her babies better and bond to them. This can explain weird sense of smell during pregnancy as well as a desire for unusual foods. Basically, olfactory system gets heavily renovated to improve accuracy.

Baboons distinguish social implications of dominance reversal depending on family relatedness (if rank 4 submits to rank 27 - others have no interest if 4 and 27 are from the same family - “crazy relatives”).

When mating, one has contradicting goals - prevent inbreeding, but to pass as much of their own genes as possible. As a result, mating with the 3rd cousin (i.e. two of your grandparents are siblings, i.e. had the same parents) is the optimal solution. This is observed in nature. Humans also find the odor of 3rd cousin the most appealing.

Non innate recognition

Non innate recognition requires imprinting, i.e. learning who’s mother / baby. The fact that learning occurs is innate. One can learn sounds, odor or appearance.

Goats don’t have innate ability to recognize relatives. Instead there are other approximations:

  • “smells like my vaginal fluids”
  • lick the kid and “smells like my saliva”
  • scent mark and “smells like me”
  • mouth smells like my milk.

This can be used to detect siblings as well, e.g. “smells like my mom vaginal fluid”. This can be proven by cross fostering, the goat will take longer to decide (“hm, it does not smell that much like my vaginal fluid”), but in the end will take care of the kid anyway (“but there is no other baby, so yeah, it must be mine”).


Humans do it cognitively. Hey, I see this one baby around me, it must be the one I just gave birth too. Who was the father? The only guy with whom I had sex. To detect sibling - “I saw mom giving birth to them”.

Baboons can also do this. Males care about kids more when they are sure. E.g. “if I am the only one who mated with her” (for example, female can be too young and not that fertile, thus, no interest from higher rank baboons and, as a result, mate with exactly one lower rank baboon). They also consider probabilities - the closer to female’s prime fertility point a baboon mated with her, the more likely he is to take care of the kids.

This works even in fish. In sunfish males are very paternal. If you bring another male (e.g. in an transparent container) between a female and a male (closer to the female), the latter male will assume that he is not the father.

Fusiform cortex is used to recognize faces. Normally it fires more for relatives (e.g. mom > stranger > chair). In autistic people it does not work well (mom ~ stranger ~ chair). Sheep and pigeons have it too.

Babies can distinguish mom smell after birth (but not dad’s) - when given two sources of smell, they prefer one with mom’s smell. The can recognize mom’s voice (by learning it from inside), but not dad’s (hard to hear it from inside).

How does this affect your decision who to mate with? One study considered Kibbutz in Israel, a collective community, which raises their kids all together. If two kids were raised in the same age group up to 6 years of age, they never marry each other. In other words, if it is someone, who you spend a lot of time before 6 years of age, you don’t mate with them (they are more like a sibling).

There are ways to manipulate us to feel more/less related than we are (pseudo kinship / pseudo speciation). Military does this a lot (us - brothers, them - non-humans).