Sisters Choosing Him For Themselves
Research shows that women value physical attractiveness more when choosing a partner for themselves than for a sister.
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 was inspired by earlier work on parental-conflict over mate choice. In many cultures, parents choose spouses for their children and do not always use the same criteria as their kids do. Although women and their mothers agree strongly on what constitutes a good partner (for example, it’s important than a man is responsible, but not important that he is a dreamer), there are areas of disagreement. For themselves, women desire a partner who is sexy, charming, and funny; 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 over their offspring’s choices. But sibling rivalry is every bit as potent as parent-offspring conflict. So, how and why do siblings disagree about what makes a perfect partner?
The research
Biegler and Kennair recruited ~300 sisters to participate in 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’ (e.g. supportive, nice, friendly, sociable) and that were more related to ‘sexiness’ (e.g. thrill-seeking, seductive, charming).
Women generally thought it more important that a partner was agreeable than sexy, but deeper analysis showed that preferences differed depending on whether women were judging for themselves or their sister. Women did want an agreeable partner for themselves and their sister, but would rather have a sexy spouse than a sexy brother in law. Spouses should be charming, passionate, and playful; brothers in law should be sensible and know how to set limits.
It’s worth pointing out that sisters disagreed on traits that they thought were generally important and unimportant. Sisters didn’t really want a partner who who knew “how to set limits” for themselves or for their sister, but they knew they would rather have a brother in law than a spouse who was handy with limit-setting. Again, women thought it important that a partner be sexually satisfying, but they preferred to keep a sexy man to themselves rather than share him with their sister.
Why the sibling rivalry?
So, despite the trend toward agreement, why do siblings disagree over how sexy their 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 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 partner [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 a disagreeable deadbeat misanthrope.
The surprise here is that, even though siblings are not thought of as having a strong impact on one another’s dating partners, the evidence suggests that women are evolutionarily adapted to meddle in their sisters’ love lives.
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
For an audio version of this story, see the 9 February 2016 episode of The Psychology of Attractiveness Podcast.
