Oh, Brother: What Do We Find Attractive In A Romantic Partner?

The question of what we find attractive in a romantic partner is a curious one. Why are hourglass figures, a clear complexion, or a toned physique so enticing? Some evidence suggests that we have evolved over time to find these traits attractive as they denote a benefit to the beholder¹ — youth, fertility, parasite or pathogen resistance — and this signals “good” genes for your potential offspring².

While we may no longer live in the times of cavemen, these cues are still helpful to us, after all, who doesn’t want the best for their children? We may have adapted to choose partners wisely based on their visible cues, but there are still individual differences between us all. We see a lot of assortative mating, that is, a partner with similar traits to yourself, whether that be the level of attractiveness (attractive women tend to end up with attractive men³) intelligence⁴, religiosity⁵, and even height⁶.

Research also finds that we may like parent-similar traits. For example, much research has found links between an individual’s opposite-sex parents’ eye color and their partner’s eye color⁷ (mum has blue eyes, wife has blue eyes, too). The theory behind this seems like a social learning mechanism: the idea that as children, we make a template based on our parents, and unconsciously look for a partner that matches that template⁸.

We heard about this concept and wondered if this imprinting-like effect extended to siblings as well as parents. We recruited 32 female participants with a brother and a boyfriend who both sent photographs of themselves. We also used 48 celebrity females’ brothers and boyfriends, with recent photographs. We divided the photographs into sets of four brothers and four boyfriends, keeping the celebrity and volunteer samples separate (due to the difference in the context of the images). We arranged the sets in tableaux following previous work, with one brother on the left-hand side and four boyfriends on the right. We then asked 32 women to rank how similar the four men on the right (the partner plus three foils) were to the one on the left (the brother) to investigate female perceptions of similarity, and none of the women knew that there was a link between any of the men. Each of the 32 women rated all 56 tableaux in a random order, and no one guessed the true aim of the study.

We found that women do in fact appear to select partners who resemble their brothers⁹. When we look 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, this only applies to our dataset.

In order to try to generalize our results, we used a statistical model to try to predict probabilities. Our model indicated that if we generalized beyond our dataset, people would select 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 predicted that people would say that a woman’s boyfriend and her brother looked least alike just 16% of the time. These effects may seem subtle, but this subtle effect is exactly what we would expect in psychology: partner choice is such a tangle of decisions, factors, and influences, along with a good helping of chance.

So what does all of this mean? To be clear, we do not look for the exact replicas of our brothers or parents, we look for a resemblance. This is not about incest or Freudian complexes; in fact, we actively avoid incest due to the inbreeding consequences to offspring¹⁰. A little resemblance is good for reproductive success! A famous Icelandic study by Helgason and colleagues actually found that when looking at all known couples born in Iceland over a 165-year period, the most reproductively successful couples were those who were related at around the third or fourth cousin¹¹. It seems that the “optimal” partner is one who is related enough so that they have the appropriate genetics to survive the environment, but not so related that the consequences of inbreeding occurs¹².

These findings are described in the article entitled Facial resemblance between women’s partners and brothers, recently published in the journal Evolution and Human Behavior. This work was conducted by Tamsin K. Saxton, Catherine Steel, Katie Rowley, and Amy V. Newman from Northumbria University, and Thom Baguley from Nottingham Trent University.

References:

1.  Buss, D. M. (1989). Sex differences in human mate preferences: Evolutionary hypotheses tested in 37 cultures. Behavioral and Brain Sciences, 12(1989), 1–49.
2.  Zahavi, A. (1975). Mate selection-A selection for a handicap. Journal of Theoretical Biology, 53(1), 205–214. https://linkinghub.elsevier.com/retrieve/pii/0022519375901113
3.  Feingold, A. (1988). Matching for Attractiveness in Romantic Partners and Same-Sex Friends : A Meta-Analysis and Theoretical Critique. Psychological Bulletin, 104(2), 226–235.
4.  Watson, D., Klohnen, E. C., Casillas, A., Simms, E. N., Haig, J., & Berry, D. S. (2004). Match makers and deal breakers: Analyses of assortative mating in newlywed couples. Journal of Personality, 72(5), 1029–1068. https://onlinelibrary.wiley.com/action/cookieAbsent
5.  Ellison, C. G., Burdette, A. M., & Bradford Wilcox, W. (2010). The couple that prays together: Race and ethnicity, religion, and relationship quality among working-age adults. Journal of Marriage and Family, 72(4), 963–975. https://onlinelibrary.wiley.com/action/cookieAbsent
6.  Stulp, G., Simons, M. J. P., Grasman, S., & Pollet, T. V. (2017). Assortative mating for human height: A meta-analysis. American Journal of Human Biology, 29(1), 1–10. https://onlinelibrary.wiley.com/action/cookieAbsent
7.  Little, A. C., Penton-Voak, I. S., Burt, D. M., & Perrett, D. I. (2003). Investigating an imprinting-like phenomenon in humans partners and opposite-sex parents have similar hair and eye colour. Evolution and Human Behavior, 24(1), 43–51. https://linkinghub.elsevier.com/retrieve/pii/S1090513802001198
8.  Debruine, L. M., Jones, B. C., Little, A. C., & Debruine, L. (2017). Positive sexual imprinting for human eye color, 1–8. https://www.biorxiv.org/content/10.1101/135244v2
9.  Saxton, T. K., Steel, C., Rowley, K., Newman, A., & Baguley, T. (2017). Facial resemblance between women’s partners and brothers. Evolution and Human Behavior. https://linkinghub.elsevier.com/retrieve/pii/S109051381630280X
10.  Marcinkowska, U. M., Moore, F. R., & Rantala, M. J. (2013). An experimental test of the Westermarck effect: Sex differences in inbreeding avoidance. Behavioral Ecology, 24(4), 842–845. https://academic.oup.com/beheco/article/24/4/842/220309
11.  Helgason, A., Pálsson, S., Guðbjartsson, D. F., Kristjánsson, P., & Stefánsson, K. (2008). An Association Between the Kinship and Fertility of Human Couples. Science, 319(February), 813–816. https://science.sciencemag.org/content/319/5864/813
12.  Bateson, P. (1983). Optimal Outbreeding. Mate Choice, 257, 277.