Voyagerix/Shutterstock
Source: Voyagerix/Shutterstock

Psychologists from Norway have discovered that sisters disagree over what makes a person attractive, and that their answers depend on whether they are choosing a partner for themselves or for their sibling.

Robert Biegler and Leif Edward Ottesen Kennair’s research took inspiration from earlier work on parental-conflict over mate choice. In many cultures, parents choose spouses for their children—but do not always use the same criteria as their kids would. Although women and their mothers in such groups may strongly agree what constitutes a good partner—for example, that it's important that a man is responsible, but not that he is a dreamer—there are areas of disagreement. For themselves, women desire a partner who is sexy, charming, and funny; choosing for them, their mothers want a son-in-law who is disciplined, industrious, and honorable.

Siblings traditionally exert less influence over one another’s choice of mates than parents do. But sibling rivalry is every bit in this area as in others. So how and why do siblings disagree about what makes a perfect partner?

The Research

Biegler and Kennair recruited approximately 300 sisters for their research. These women judged how important it was that a potential long-term partner possessed each of 133 traits, thinking first of a partner for themselves, and then of a partner for their sister.

The traits were split into two groups—those that were about ‘agreeableness’ (supportive, nice, friendly, sociable) and that were more related to ‘sexiness’ (thrill-seeking, seductive, charming).

Overall, the women generally thought it more important that a partner was agreeable than sexy, but deeper analysis showed that their preferences differed depending on whether they were judging for themselves or for a sister. Women chose many agreeable characteristics for themselves and a sister, but clearly preferred a sexy spouse to a sexy brother-in-law. (See chart below.) Their won spouses should be charming, passionate, and playful; as for their sisters', they should be sensible and know how to set limits.

Robert Burriss, redrawn from Biegler and Kennair (in press)
Source: Robert Burriss, redrawn from Biegler and Kennair (in press)

It’s worth pointing out that sisters disagreed on traits that they thought were generally important and unimportant. Women didn’t really prioritize a partner who knew “how to set limits” for themselves or for their sisters, but they knew they would rather have a brother-in-law than a spouse who was strong on limit-setting.

And while women thought it important that a partner be sexually satisfying, but they preferred to keep such a man to themselves rather than offer him to a sister.

Why the Rivalry?

Why do siblings disagree over how sexy each other's partners should be?

The reasoning goes like this: We share 50% of our genes with our full siblings. Because the game of life — evolutionarily speaking — is to pass our genes on to the next generation, we should all be invested in what our siblings do with the genes we share. Just as I want to pass on my own genes, I should also want my sibling to pass on her genes because half of them are identical to mine. However, because we only share half our genes, I should probably just care half as much about her evolutionary impact as I do my own.

This is relevant when it comes to judging the importance of a partner’s sexiness because sexiness is thought to be heritable—an indicator of good genes. If we pair up with a sexy spouse, we can transmit the benefits of those good genes to our offspring. But a sexy sibling-in-law is less appealing. As Biegler and Kennair point out in their paper:

The benefits of any good genes [that the partner of our sibling] may have are (normally) not available.

This is the same logic that is used to explain parent-offspring conflict over mate choice: We also share 50% of our genes with our parents and offspring, hence parents preferring sensible over sexy sons- and daughters-in-law. An agreeable in-law can benefit the whole family, or at least pose less of a drain on the family’s collective resources, than an attractive but disagreeable deadbeat or misanthrope.

The surprise here is that, even though siblings are not thought to have a strong impact on each other’s dating partners, the evidence suggests that women are evolutionarily adapted to meddle in their sisters’ love lives.

Reference

Biegler, R., & Kennair, L. E. O. (2016). Sisterly love: Within-generation differences in ideal partner for sister and self. Evolutionary Behavioral Sciences, 10(1), 29–42. Read summary

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