Casual Sex or Wedding Bells? -- Bohannon 2005 (323): 5 -- sciencenow:
Decades of research indicate that people tend to trust those with faces similar to their own and are even more likely to marry them. Evolutionary theory provides an explanation for the first tendency: we are wired to recognize and help our kin because they are likely to share our genes. But evolution should also protect us against the dangers of inbreeding by preventing us from hopping in the sack with close relatives.
To find out how these conflicting motivations play out, Lisa DeBruine, a psychologist at St. Andrews and Aberdeen Universities, Scotland, presented 144 college students with 36 pairs of computer-generated faces. The faces in each pair were of the same race and the opposite sex as the viewer, but one was manipulated to have facial features more similar to those of the viewer.
As in earlier studies, the students found faces more trust-worthy if they were like their own. When it came to the prospect of a one-night stand, students found facial similarity a turn-off, DeBruine reports online today in the Proceedings of the Royal Society B: Biological Sciences. She concludes that people are less sexually attracted to individuals who look like kin in order to avoid inbreeding.
Evolutionary hypothesis tested.