A mental mechanism that allows close kin to be perceived as inappropriate mating partners.

This effect accounts for the fact that, to our best knowledge, in every human society in history, incest is avoided and usually a social taboo
Children who were raised together develop a strong sexual disinterest for each other. This even works if they are not biologically related.
The effect has been studied in Israeli kibbutzim, where children are raised in cohorts of the same age, and there are hardly any marriages among members later.
The causes are somewhat unknown. Supporters of Evolutionary Psychology suggest that genes which would cause such a behaviour would be selected for, since children from incestuous relationships have higher chances of hereditary diseases and infertility.

Weingart et. al. (eds.) (1998), "Human By Nature". Mahwah, Lawrence Erlbaum Associates