Women Date Men Who Look Like Their Brothers, Study Says

Evolutionary theory suggests as much. According to research on optimal mating, reproductive success is a delicate dance between avoiding incest and at the same time avoiding people who are too genetically dissimilar to you. It's well-known that inbreeding is problematic, but scientists say that excessive "outbreeding" also has genetic consequences. Thus people have evolved, the theory goes, to seek out a mate who is right in the sweet spot.

"Previous research has already shown that people seem, on average, to pick partners who look somewhat like their parents," Tamsin Saxton, an associate professor at Northumbria University who worked on the study, told Broadly in an email. It has been theorized that parental facial features are a handy point of reference for genetic similarity. "We realized that [this theory] could also apply equally to siblings. So, we wanted to test whether we could detect perceptual similarity between someone's partner and siblings."

To do this, the researchers asked 32 volunteer women aged 19 to 40 to send in pictures of their brothers and their partners. All the women were white to control for perceived cultural similarity. For an additional sample set, the researchers collected 48 photos of the brothers and partners of public figures and celebrities.

They asked a separate group of raters to come in and match up the photos based on who they thought looked alike. The group wasn't aware of that the people in the photos had a relationship to each other. The volunteers saw a photo of one woman's brother, for example, and picked a photo out of a lineup of four other men that looked the most similar, the second most similar, and the third most.

The researchers found that just under one third of raters picked a woman's partner as the most similar to her brother. "If the volunteers were picking entirely by chance, then we'd expect one quarter of their choices to be for the 'correct' brother-boyfriend pair," Saxton said.

A more sophisticated analysis of the data, controlling for age and other variables, showed that the raters picked the correct brother-boyfriend pair as most similar 27 percent of the time, which is slightly above chance. In other words, the study found that it's more than a coincidence that women picked boyfriends that looked like their brothers.

But Saxton says you might not want to narrow your relationship search to guys who look like your brother just yet. "One important point is that you shouldn't expect to be able to pick a partner simply based on the appearance of a sibling! Not all women had partners that looked like their brothers," she said.

"The point though is that it's pretty weird to think that our partners might bear any resemblance whatsoever to our brothers—and an everyday prediction might be that partners and brothers wouldn't look alike at all," Saxton added. "However, our study found that there was this subtle resemblance."