How Your Family Influences What You Find Attractive

Psychologists from the UK show that romantic and filial love are not as separate as we might like…

The less said about how the Corleone brothers dealt with their new brother-in-law, the better…

Humans are a social animal, and it is in our nature to crave the affection of our partners and our family members, and to return that affection in kind. But as much as we desire romantic and filial love, the prospect of mixing the two fills most of us with revulsion.

In some cultures, parents arrange the marriages of their offspring; in the West we tend to prefer to choose our own partners, and resist our parents interfering with this most personal of decisions. And very few of us, regardless of our cultural background, would countenance perhaps the ultimate sexual taboo: a relationship with a sibling.

But two studies, recently published in the psychology journal Evolution and Human Behavior, suggest that family relationships and what we lust after in a sexual partner are more intertwined than many would care to imagine.

The first study was led by Tamsin Saxton (whom I should point out, for reasons of full disclosure, is a former colleague of mine at Northumbria University). Saxton and her colleagues wondered if women tend to choose male partners who resemble their own brothers.

And it turns out that this is not so peculiar a theory as it may sound. This is because there is a wealth of research showing that individuals, across a wide range of species, are more reproductively successful (they produce more offspring) if their mate is not too genetically similar or dissimilar. If we mate with someone who is very genetically similar to us, for example a sibling (with whom we share 50% of our genes), we run the risk of concentrating harmful recessive alleles in the next generation (case in point: the famously inbred royal families of Europe in the 19th century, among whose members the genetic condition haemophilia was common). However, if our partner is too genetically different, we risk separating out genes that work well together or are adapted to our local environment. We also increase the costs of altruism: previous research shows that we are friendlier towards family members to whom we are more closely related, and relatedness between family members is diluted when parents share fewer genes.

Therefore, humans (and other sexually reproducing organisms) have a problem: how to find a partner who is genetically similar but not too similar.

The well-known Westermarck effect prevents us from developing a sexual attraction to our siblings. If we grow up with another person in the same home, we tend to find the idea of sexual intercourse with that person extremely aversive. These people will usually be siblings or other close relatives, although raising unrelated children in the same household (e.g. in a kibbutz or in the home of a blended family) can have the same effect. But do we find ourselves drawn to potential partners we know are not our siblings, but who nevertheless look a bit like them? Saxton decided to find out.

She asked a group of young women to contact their brothers and boyfriends. These men sent Saxton photos of themselves. Saxton then used these photos to create an unusual set of family albums. On the left side of each page she placed the photograph of a woman’s brother; on the right side of each page she placed the photograph of the same woman’s partner, but surrounded by three randomly selected photographs of other women’s partners.

A new group of female volunteers was given the task of perusing these photo albums and, on each page, ranking the four partners for their similarity to the brother. It goes without saying that these new volunteers were not made aware of the purpose of the experiment. They didn’t know the photos were of brothers or partners. As far as they were concerned, these were all randomly selected and unrelated men.

Because each page of the albums presented four possible choices (the volunteers could choose any of the four partners as the closest physical match for the brother), the odds that the volunteer would select the correct partner as a match for the brother were 1 in 4, or 25%. However, Saxton found that women were slightly more likely than chance to rank the correct partner as the best match: 27%. The volunteers were also more likely than chance to rank the correct partner as the second best match (32%) or third best match (26%); the correct partner was thought to look the least like the brother only 16% of the time.

These results suggest that women are not choosing male partners who look indistinguishable from their brothers, but that there is a “family resemblance” between brothers-in-law above and beyond what we would expect by chance.

However, it is still unclear whether women actually prefer men who are their brothers’ doppelgangers, or if they simply end up with a brother-resembling man for other reasons. For example, perhaps women from certain socioeconomic backgrounds tend to choose a partner from the same background. This could explain the results if people from the same background just happen to look similar.

You might also wonder if we would find the same effect in men: do men choose female partners who resemble their sisters? Saxton and her colleagues speculate that this is likely, and that the effects might even be stronger that those they observed for women:

“Men might find sibling-resemblance less aversive than women do, and this can be explained by men’s lesser reproductive investment and hence lower risk in a sub-optimal partner”.

In other words, women have to carry a baby to term and they tend to take the lion’s share of childrearing duties, while men are more likely to abandon their offspring. So men have less to risk from mating with a very closely related person. Now there’s a conversation topic for your next family gathering…

Today’s second study, coincidentally co-authored by the prolific Tamsin Saxton with our former colleague Carmen Lefevre, is about parents’ preferences for physical attractiveness in prospective sons-in-law.

In the West, arranged marriages are not very common. Nevertheless, only the most restrained of parents offers no opinions on the love lives of their children. Many of us have experienced our parents passing judgment on our boyfriends or girlfriends, or even forbidding us from seeing certain people. From the perspective of the offspring, this interference can seem inexcusable. Parents should kind their own business! But on an evolutionary level, it makes perfect sense. Parents are biological organisms, and like any other organism are concerned with the propagation of their genes (even if this concern is not consciously expressed in such blunt terms). As offspring are the only conduit for these genes, parents should be interested in who their offspring choose to reproduce with.

However, as previous researchers have pointed out, parents should value some traits more than others in their potential sons-in-law. A man can provide his partner with both material investment (money, food, affection) and a contribution to the shared genetics of their offspring. Because a woman will share 50% of her genes with her offspring, but only 25% of her genes with her grandchildren, we should expect women to value “good genes” in a male partner more than in a son-in-law. A woman simply has “more skin the game” (or should that be blood?) when choosing a partner than a son-in-law.

But when it comes to judging a man for his willingness to materially invest, we should expect women to value this trait more in sons-in-law than in partners for themselves. A son-in-law who invests in your daughter decreases your own investment burden. If you don’t have to invest as much in this daughter — because your new son-in-law in picking up the slack — you will have more time, energy, and resources to invest in your other children and grandchildren.

Research has provided support for these theories: we prefer good looks in a partner more than in a son- or daughter-in law, and a good family background more in an in-law than in a partner. But Lefevre and Saxton wondered to what extent parents would prefer various indicators of a man’s genetic quality in potential sons-in-law, compared to the preferences of their daughters.

Lefevre and Saxton recruited 87 young women, who in turn persuaded both their parents to join them as participants in the study. The researchers used computer graphics software to create images of men’s faces that varied on four indicators of genetic quality: facial attractiveness, masculinity, healthy skin color, and symmetry. The participants’ task was to look at pairs of faces differing on each of these four traits and, for each pair, to pick which face was more attractive. Daughters picked the men they preferred as a partner for themselves; parents picked the men they preferred as a partner for their daughter.

Lefevre and Saxton found that daughters wanted a man who was more facially attractive than the ideal man picked by their fathers, while daughters and mothers were roughly in agreement as to the importance of facial attractiveness. The pattern for masculinity preferences was much the same: daughters and their mothers preferred a similarly masculine man, while fathers preferred their daughters shacked up with a slightly less macho chap. When it came to a healthy appearance, daughters valued this trait more than both of their parents did, but all three family members valued a symmetrical partner for the daughter to a roughly similar extent.

These results show that parents are less interested in “good genes” indicators for their daughters’ partner than the daughter is herself, and that men are less interested than their spouses are. Nevertheless, the differences between parents and offspring and not especially pronounced.

In an interesting follow up analysis, the researchers found that mothers’ preferences for their daughter’s partner were no different if the mother had or had not gone through the menopause. This is striking because previous research has shown that the menopause changes the kind of man a woman is most attracted to. After the menopause, women are less interested in markers of genetic quality in a man. This new research suggests that menopause does not disrupt a woman’s ability to distinguish attractive young men from their less handsome peers: when judging a man’s suitability for their daughter, women are just as discerning after their menopause as before. Perhaps something to bear in mind before you invite your next boyfriend home to meet the parents.

Lefevre, C. E., & Saxton, T. K. (in press). Parental preferences for the facial traits of their offspring’s partners can enhance parental inclusive fitness. Evolution and Human Behavior. Read summary

Saxton, T. K., Steel, C., Rowley, K., Newman, A. V., & Baguley, T. (in press). Facial resemblance between women’s partners and brothers. Evolution and Human Behavior. Read summary

For an audio version of this story, see the 6 June 2017 episode of The Psychology of Attractiveness Podcast.

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