Why Don’t You Have Sex With Your Sister? - Dr Debra Lieberman - #1041

**Chris Williamson** (0:00)
Why don't people want to have sex with their sister?

**Debra Lieberman** (0:04)
Or their brother, or other family members? It ends up that humans have a natural inbreeding avoidance system that develops pretty reliably in most folks exposed to the cues, which I term kinship cues, that are available during childhood.

**Chris Williamson** (0:24)
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?

**Debra Lieberman** (0:41)
This is a really good question. It's always fun to open up an interview with incest avoidance.

**Chris Williamson** (0:47)
Incest avoidance, yeah.

**Debra Lieberman** (0:48)
Yes. 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 they're imperfect. And so even though we could join us sorority or fraternity and call people brother, sister, or in certain religions, father, mother, you know who your actual father and mother are. You know, well, with good certainty, you know who your mom is, who your daddy is, is always an interesting question. But certainly siblings are also individuals who you tend to correctly identify, or would have at least in ancestral environments. So we don't use language. Likely, we just use the cues that correlated. And so if you want to go around the horn of the nuclear family to figure out, how do we detect who our close genetic relatives are? Well, how do you know who your mother is? This is the female who tended to primarily 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, yes, it falls out of you, unmistakable. How does a child know who her father is? This is not pinned down. I mean, it is a question. I mean, mommy's baby, daddy's maybe. So the question about how it is that people, how it is people identify their father really probably goes through understanding who is, have I correctly identified my mother and what males are investing in her because males are going to rely on their paternity certainty in order to direct investment towards their offspring. So relying on what kind of, who's investing in me heavily out there could be one cue that females use, but also the male that was around very early on in development has a better chance of being one's dad than the male that one's mom kind of is hanging out with later on in life. So, but it's a really, it's a really good question. So might phenotype matching occur? It could be. I don't, I'm not a fan. So sorry, that is, do people use facial resemblance as a cue to understand who's likely a relative?
That hypothesis does have some interesting data backing it. I'm not a fan of it only because I think ancestrally people looked really similar. Not only that, just by the assortment of genes, siblings can look and fathers can look a lot like their offspring or not as much like their offspring, but still be related with good certainty. So I'm not quite sure pinning how we determine kinship onto a few genes that happen to govern facial features is the right way to do it. But maybe it contributes to be sure. Certainly, when we see people who look similar to us, that can spark a type of kinship. But I think that has more to do with a friendship.