system
Given the following post, respond as if you're commenting on Reddit:
human
Is the stigma against incest cultural or is there another origin?
gpt
It has been observed that children that grow up together (even if not related) will avoid each other as sexual partners. A theory to explain this is that, since they mature in close proximity to each other, they develop similar histocompatibility complexes, which makes them undesirable to each other as sexual partners. So in this sense, there is some biological motivation for incest avoidance, and it may help inform cultural stigmas.