A new study suggests that when it comes to dating, women tend to pick partners who bear at least a slight resemblance to their own brothers. Reporting in the journal Evolution and Human Behavior, the researchers write that selecting "optimal" breeding mates—not too closely related, but not too distantly, either—appears to be something we humans do, and though they haven't tested it yet, they suggest that men likely do the same. In the study, the researchers had women submit photos of their brothers and boyfriends, then added photos of three other men to the "boyfriend" mix. They had strangers choose which of the boyfriend photos most closely resembled the brother. The strangers picked the actual boyfriend 27% of the time, which Discover sees as a "small but statistically significant" improvement over mere chance.
"We have such a strong aversive reaction to thoughts of anything even remotely resembling incest that you might assume there would be no way that a woman’s partner could resemble her brother—but the study shows this not to be the case," one of the researchers tells PsyPost. That said, she notes that the tendency is only subtle and needs to be studied further. Discover explains that there might be biological reason behind the preference: While incest carries all kinds of genetic risks, "some amount of genetic similarity between parents can actually be evolutionarily advantageous." It's what the researchers call "optimal outbreeding." (This woman married her son, and later her daughter.)