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

Or their brother or other family members.

It ends up that humans have a natural inbreeding avoidance system that develops pretty reliably and most folks exposed to the cues, which I term kinship cues, that are available during childhood. 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 an interview with incest, incest avoidance. Incest of you, yeah. 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? 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 less healthy offspring, for instance, and offspring who might suffer from greater genetic

mutations.

So evolution engineered into our psychology, a very sophisticated system to allow us to detect

relatives, close genetic relatives, and develop a sexual aversion towards them. We don't even

typically think of them as possible mating partners. It's not foolproof, but this is what tends to

happen. And 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 genes by common

descent. But you ask the question of, so how do we do this? And because 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 in that species' evolutionary history.

So whether it's being part of the same litter or a smell or imprinting on a particular place or a marking, these are the kinds of things that evolution can engineer to help guide kin detection.

Humans have language, and so language and culture map very nicely onto these systems, but

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