Research published in the scientific journal Evolution and Human Behavior suggests that women tend to pick partners who show some slight resemblance to their brothers.

“Previous research has already shown that people seem, on average, to pick partners who look somewhat like their parents,” explained Tamsin K. Saxton of Northumbria University, the study’s corresponding author. “(I wrote a short summary of that research here). A few theories have been put forward to explain why people might do that. But we realised that most of these theories could also apply equally to siblings. So, we wanted to test whether we could detect perceptual similarity between someone’s partner and siblings.”

By examining 64 facial photographs, the researchers found evidence that women’s romantic partners tended to resemble their brothers. Though they have yet to test this with a male sample, they believe the same tendencies should apply to men as well.

“One focus of our research is to understand more about how people pick the partners they do, and why people differ in their preferences and choices,” Saxton explained. “Recently, we’ve been trying to unpick the influences that people’s families might have on their partner preferences. We predicted that sibling resemblance might be just one of the many contributing factors in partner choice, and the current study indicates that it may indeed have an influence.”

“Having said that, it’s important to note that the effect is subtle. The way the study was designed was that we showed photos to the volunteers who did the ratings. The volunteers saw a facial photo of a man on the left-hand side of a sheet of paper. This man was a woman’s brother (though the volunteers didn’t know that). Then, on the right-hand side of the same sheet of paper, they saw facial photos of 4 other men. Again, the volunteers didn’t know that one of the men was the boyfriend of the same woman. We asked the volunteers to say which of the men on the right-hand side was most, second most, third most, and least similar to the brother.”

“If there was no similarity at all between a woman’s brother and partner, then we’d expect the volunteers to pick randomly, selecting each of the four pictures one quarter of the time,” Saxton said. “When we looked just at the raw numbers, we found that nearly one third of the raters’ choices were for the ‘correct’ brother-boyfriend pair as looking most similar. However, these raw numbers are only indicative, and we wanted to know how we might extrapolate the data to the population at large. We used a statistical model to try and predict this. Our model indicated that if we generalised beyond our dataset, we came up with a selection rate for picking the ‘correct’ brother-boyfriend pair as most similar 27% of the time, and as first or second most similar combined 59% (instead of 50%) of the time. The model gave a selection rate of just 16% for picking the brother as least similar to the boyfriend. Psychologists are interested even in these subtle effects because behaviour is complicated and arises from a multitude of different influences.”

Saxton emphasized that the effect of sibling resemblance appeared to be small — and only one factor among many that influences attractiveness.

“So, I guess one important point is that you shouldn’t expect to be able to pick a partner simply based on the appearance of a sibling,” she explained to PsyPost. “Not all women had partners that looked like their brothers. There will be many relationships where a woman’s partner doesn’t look like her brother, and that’s entirely consistent with what we found. The point though is that we might anticipate intuitively that there could be no resemblance between partners and siblings. However, our study found that there was this subtle resemblance, on average across the sample.”

Saxton also noted that her study had some caveats and alternative explanations.

“We realise of course that people’s parents resemble their siblings. So it’s possible that our ‘finding’ of brother-boyfriend resemblance could be an essential corollary of parent-partner resemblance. However, first, I think it’s theoretically plausible at least that you could pick a partner who looks like your parents but not your siblings (although this remains to be tested). Second, your siblings might be around even when your parents aren’t. Third, in the published paper, we compared our findings with previous work on parent-partner resemblance. We found that the effect size was broadly similar in the previous work compared to ours. Finally, I guess the point is that 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.”

“Also – it’s not clear how attraction and actual partner choice might map onto each other – they are not necessarily identical things,” Saxton explained. “Indeed, an earlier research paper used computer software to generate facial images that looked a little like the siblings of the participants in the research study. Then they asked the participants to rate how attractive the facial images were. The researchers found that men rated sibling-resembling faces as significantly more attractive than self-resembling faces but not significantly different from control faces, whereas women rated sibling-resembling faces as significantly less attractive than control faces and no different from self-resembling faces. Attractiveness judgements assessed in a laboratory context may not always map directly onto patterns of relationship formation and maintenance when other factors come into play.”

“Another caveat is that the photographs that we used weren’t fully standardised; they portrayed facial features, but also facial expressions, hairstyles, and some elements of clothing and background. We asked the raters to judge facial similarity, but these non-standardised elements likely contributed somewhat to their decisions. Therefore, it is possible that our raters were not matching the photographs merely on facial structural similarity, but also on elements such as emotionality (perceived through facial expressions), and socio-economic status and cultural cues (perceived through clothing and hairstyle).”

“Finally, our study focussed on contemporary western populations,” Saxton told PsyPost. “We found consistent results irrespective of whether we focussed on the photos that we had collected through direct contacts, or those that we had sourced online from the people more in the public eye. We haven’t tested whether the results would hold outside of these social groups though. In fact, we found a lot of variability in the extent to which a woman’s partner and brother resembled each other. If there are systematic influences that lead women away from a partner who looks like her brother, then in a context or cultural setting where those influences are greater, we might not find the same effects.”

The study, “Facial resemblance between women’s partners and brothers“, was also co-authored by Catherine Steel, Katie Rowley, Amy Newman, and Thomas Baguley.