Why Don’t You Have Sex With Your Sister? - Dr Debra Lieberman

Why don't people want to have sex with their sister?

(laughs) Or their brother. (laughs)

Or their brother.

Or other family members.

Yep.

It ends up that humans have a natural inbreeding avoidance system that develops pretty reliably in most folks exposed to the cues, uh, which I term kinship cues, uh, that are available during childhood. Um...

Right. Well, what about animals? Because I understand there is this label, that's your brother, that's your sister. How do animals actually detect who their relatives are? We just take it for granted in humans, you can point and say. But animals don't have language, so how do they know?

This is a really good question. It's always fun to open up a interview with incest. (laughs) Incest avoidance.

Incestoview, yeah.

Um, yes. And so, so it's a really good question. How do we know who our close genetic relatives are and why is that important? Um, well first, it's important to know who your close genetic relatives are not only for the purpose of not mating with them, because mating with close genetic relatives can cause a host of problems. So it leads to, uh, less, um, less healthy offspring, for instance, and offspring who might suffer from greater genetic mutations. Uh, so evolution engineered into our psychology a very sophisticated system to allow us to detect a- uh, detect relatives, close genetic relatives, and develop a sexual aversion towards them. We don't even typically think of them as possible, uh, mating partners. It's not foolproof, uh, but this is what tends to happen. And ano- the other reason why we should have a system for naturally detecting and automatically detecting genetic relatedness is for altruism. So being nice to your close genetic relatives follows from Hamilton's theory of inclusive fitness. We're nice to other people as a function. One way altruism can evolve is by being nice to people who tend to share, uh, genes by common descent. But you asked the question of, so how do we do this? Um, and 'cause humans aren't the only species that encountered this problem of avoiding mating with close genetic relatives for biological purposes. And so other animals without language or culture, they use cues, cues that correlated with another individual being a close genetic relative, uh, i- in that species's, uh, you know, evolutionary history. So whether it's being part of the same litter or a smell, um, or imprinting on a particular place or a marking, these are the kinds of things that evolution h- can engineer to help guide kin detection. Humans have language, and so language, uh, and culture map very nicely onto these systems. Um, but it, they're imperfect, and so even though we could join a sorority or fraternity and call people brother, sister, or in certain religions, father, mother, uh, you know who your actual father and mother are. You know, well, with good certainty you know with your m- who your mom is, who your daddy is is always an interesting question. But certainly siblings are, uh, also individuals who you tend to correctly identify, or would have at least in ancestral environments. Um, so we don't use language. Likely we just used the cues that correlated. And so if you want to go around the horn of the f- nuclear family to figure out, um, how do we detect who our close genetic relatives are, well, who, how do you know who your mother is? This is the female who tended to primarily, uh, breastfeed you. So this type of imprinting or mapping onto the female who nursed you. How does a mom know who her child is? Well, that's kind of, it, yes, it falls out of you, unmistakable. Um, how does a child know who, uh, her father is? This is, uh, not pinned down. I mean, it is a question. I mean, mommy's baby, daddy's maybe.