There is thus a kind of unconscious social contagion in perceptions of attractiveness from one woman to another. This makes perfect sense from an evolutionary perspective. Copying the preferences of other women may be an efficient strategy for deciding who is a desirable man when there is a cost (in terms of time or energy) in making this assessment or when it is otherwise hard to decide. While a woman can, with a glance, assess for herself various attributes of a man that might be associated with his genetic fitness (his appearance, his height, his dancing ability), other traits related to his suitability as a reproductive partner (his parenting ability, his likelihood of being sweet to his kids) can require more time and effort to evaluate. In those cases, the assessment of another woman can be very helpful. In fact, psychologist Daniel Gilbert has shown that a woman can do a better job of predicting how much she will enjoy a date with a man by asking the previous woman who dated him what he is like than by knowing all about the man.20 This fact has been exploited for commercial purposes: there is a matchmaking website that only allows men to post if they are “recommended” by a former girlfriend.
In direct mate choice, you choose who you like, but in indirect mate choice of the sort we have been considering, you choose who others like. Indirect mate choice can even lead people to choose mates with characteristics that they did not previously care about. A slight preference by some women for men with tattoos, for example, can lead hordes of men to get tattoos and inspire other women to want men who have them.
Perhaps not surprisingly, men react differently to social information. While they clearly have shared norms about what is attractive in a woman, contextual cues in men can actually operate in the opposite way.21 College-age women were more likely to rate a man as attractive if shown a photograph of him surrounded by four women than if shown a photograph of him alone. But college-age men were less likely to rate a woman as attractive if she was shown surrounded by four men than if she was shown alone. This makes evolutionary sense: when selecting mates, males tend to be less choosy than females and so are less concerned with the opinions of anyone else to begin with. But the presence of other men conveys information of a different sort, namely, that there might be time-consuming (and stressful) competition to secure the woman’s interest.
Hence, social networks affect our relationships in two important ways. First, structural features of our position in the social network can affect whether people think we are attractive. Do we have a partner already? How connected are we? Do we have many or few partners and friends? Others notice such things about us because they say something about who we are. Second, the social network can spread ideas and change attitudes toward attractiveness. Specific preferences for the opposite sex diffuse, and both men and women come to value partners with certain appearances based on what their friends think. Of course, our friends and families also provide explicit comments on our partners and have a conscious influence on our perceptions and behaviors as well.
Unfortunately, detailed data regarding entire social networks and how sexual attitudes and behaviors spread in networks have been very scarce, and most networks that have been studied over the past century have involved only thirty to three hundred people or so. Recognizing the importance of social networks and anticipating a need for data to study them and their role in sexual behavior and other phenomena (such as youth violence, occupational success, and so on), investigators in North Carolina, including sociologists Peter Bearman, Richard Udry, Barbara Entwisle, and Kathleen Harris, designed and launched an ongoing, nationwide social-network study of American adolescents in 1994.
Known as the Add Health study, this landmark survey was administered to a whopping 90,118 students in 145 junior-high and high schools all around the United States. About 27,000 of the students and their parents were selected for follow-up surveys in 1994, 1995, and then again in 2001. Hundreds of questions were included on the survey, addressing everything from feelings about friends and family, to participation in church and school clubs, to risky behaviors like taking drugs or engaging in unprotected sex. Each student was asked to identify up to ten friends (five male and five female), most of whom—crucially—were also in the sample. The study also collected information about people’s romantic partners. All of this allowed scientists to see for the first time, very large, detailed, and comprehensive social networks and to discern the precise architecture of a person’s social ties as it changed across time. We can use these data to identify who is at the center of the network and who is at the periphery, who is located in tightly knit cliques and who prefers to associate with several different groups.
Ties between parents and their adolescent children were critical in the transmission of norms and modeling of behavior. For instance, one study that used the Add Health data showed that girls with a close relationship with their fathers were less likely to become sexually active.22 However, much more important than parents are the peers in an adolescent’s network. Add Health studies have shown that the number of friends, the age and gender of those friends, and their academic performance all affect the onset of sexual activity.23 Friends’ religiosity also affects whether adolescents report having sex, and the effect is strongest in dense social networks, where the adolescents’ friends tend to be friends with one another.24
What these studies show is that sexual behavior can spread from person to person, and the impact the network has depends on how tightly interconnected people are. But sometimes the story is more complicated. Peter Bearman and his colleague Hannah Brückner explored “virginity pledges,” a phenomenon that grew out of a social movement sponsored by the Southern Baptist Church, where teens pledge to abstain from sex, typically until marriage.25 The initial results, accounting for a range of other influences, showed that pledging substantially and independently reduced the likelihood of sexual debut. However, a much more nuanced picture emerged when the investigators looked at the effect more carefully within the social context of each school.
In a small number of “open” schools, where most opposite-sex friendships and romantic ties occur with individuals outside the school, more pledgers indeed meant delayed sexual debut. Surprisingly, though, in “closed” schools, where most ties occur inside the school, more pledgers meant a greater likelihood of sexual debut. These findings suggest that the pledge movement is an identity movement and not solely about abstaining from sex. In closed schools, adhering to this movement might be beneficial (in terms of delaying sex) when one is in the minority, but if pledging becomes the norm, the psychological benefits of a unique identity are diminished, and the effect is lost. It’s not just the pledge itself that constrains behavior; it’s whether the pledge confers a unique status. Riding a motorcycle and wearing a black leather jacket emblazoned with a skull and crossbones may give you a special identity in a place where few people own motorcycles, but in a place where everyone rides a motorcycle, it may simply mean you like to save gas.
Of course, peer norms can also increase sexual behavior. In fact, peers are more likely to promote sex than discourage it. Adolescents who believe that their peers would look favorably on being sexually active are more likely to have casual, nonromantic sex.26 Engaging in oral sex with a partner can actually make one more popular among one’s friends.27 These kinds of peer pressures assuredly underlie the changing mores regarding oral sex seen among American teenagers in the late 1990s. And related studies in adults have shown that people with more partners also have more variety in their sex lives and that they “innovate” more in terms of sexual practices.28
Romantic and sexual practices as diverse as contraceptive use, anal sex, fertility decisions, and divorce are all strongly influenced by the existence of these behaviors within one’s network. For example, in a paper entitled “Is Having Babies Contagious?” economist Ilyana Kuziemko examined eight thousand American