Freud argued that as children, members of the same family naturally lust for one another, making it necessary for societies to create incest taboos,but Westermarck argued the reverse, that the taboos themselves arise naturally as products of innate attitudes.
People tend to select mates that are like themselves, this is known as assortative mating. This holds both for physical appearances and mental traits. People commonly rank faces similar to their own as more attractive, trustworthy, etc. than average.