Opposites attract. Although we love to repeat this optimistic cliche about human nature, decades of psychological research have demonstrated that the truism isn't true. Rather, people seek out people who are just like them. This is known as the similarity-attraction effect, or SAE. Although there is slight variation in the strength of the effect, the SAE has been shown to exist in nearly every culture, from Western Europe to the remote tribes of the Brazilian rainforest. It doesn't matter where we live or how we grew up or which language we speak - we still want to spend time with people who feel similar. It's simply more comfortable.

Consider this study, conducted by Paul Ingram and Michael Morris at Columbia University. The psychologists invited a motley group of executives to a cocktail mixer, where they were encouraged to network with new people. Not surprisingly, the vast majority of executives at the event said their primary goal was to meet “as many different people as possible” and “expand their social network.” Unfortunately, that’s not what happened. By surreptitiously monitoring the participants with electronic name tags, Ingram and Morris were able to track every conversation. What they found was that people tended to interact with the people who were most like them, so that investment bankers chatted with other investment bankers, and marketers talked with other marketers, and accountants interacted with other accountants. Instead of making friends with strangers, the business people made small talk with those from similar backgrounds; the smallness of their social world got reinforced. The scientists summarize their results:

Do people mix at mixers? The answer is no...Mixer parties are supposed to free their guests from the constraints of preexisting social structure so they can approach strangers and make new connections. Nevertheless, our results show that guests at a mixer tend to spend the time talking to the few other guests whom they already know well.

What's worse, this craving for similarity - for interacting only with people who think and act in familiar ways - doesn't merely influence our behavior during cocktail mixers. Instead, it shapes our social world, constraining the reach of our personal network. This was elegantly demonstrated in a new paper by Angela Bahns, Kate Pickett and Christian Crandall at Wellesley College and the University of Kansas. The psychologists were interested in how the social diversity of college influenced the nature of social interaction. Did more diverse schools lead to more diverse friendships? Or did the opposite happen, so that a varied "social ecology" made us even more likely to seek out extremely similar people?

Their data set was clever: the researchers compared the relationships of students at a large state school - the University of Kansas campus has more than 25,000 students from all over the country - with four much smaller colleges in rural Kansas. (The median enrollment of these schools was 525 students.) Pairs of people were approached in public spaces, such as dining halls and dorm lounges, and given a short survey. They were asked for demographic information (age, ethnicity, political ideology, etc.) and about a variety of attitudes and behaviors, such as how they felt about birth control and whether they ever engaged in binge drinking. They were interrogated about their prejudices and religion, cigarette use and amount of time spent at the gym. The psychologists used their answers to generate a quick portrait of the person.

In an ideal world, being able to meet lots of different people at college would lead to a diversity of friends; we'd take advantage of the human variety on display. But that's not what happened. Bahns et al. found that students at the huge state school tended to spend time with people who were much more similar to them than students at the small, rural colleges. According to the scientists, the level of correlation between friends on the survey was higher on 80 percent of the questions at the University of Kansas, suggesting that the undergraduates were using the size of the campus to identify those who shared their precise set of beliefs, habits and attitudes. Instead of learning from people who were extremely different - who disagreed with their stance on abortion, or didn't like ultimate frisbee, or never attended football games - the students were obeying the similarity-attraction effect, sifting through the vast population to find the most homologous possible circle of friends. As the researchers put it, "the larger social contexts afford better opportunity for finegrained assortment."

This is sad on a number of levels. For one thing, the friendships were actually closer and longer lasting at the small colleges, suggesting that there is nothing intrinsically beneficial about seeking out similar people. (Opposites don't attract, but they should.) Other studies have found that having a diverse social network comes with impressive payoffs, such as this analysis of Stanford Business School graduates. (Those entrepreneurs with more "entropic" and "diverse" social networks scored three times higher on a metric of innovation, suggesting that the ability to access "non-redundant information from peers" is a crucial source of new ideas.) Despite such findings, our ancient social instincts lead us in the wrong direction, so that we end up trapped within a bubble of homogeneity. Such results also complicate the standard justification for affirmative action programs. In Grutter v. Bollinger, for instance, the Supreme Court held that universities have a "compelling interest in obtaining the educational benefits that flow from a diverse student body." In theory, that's absolutely true. But this research suggests that diversity sometimes backfires, so that a more varied student body leads to less varied interactions. Here are the scientists, clearly laying out the dismal irony of their data:

When opportunity abounds, people are free to pursue more narrow selection criteria, but when fewer choices are available, they must find satisfaction using broader criteria. Our findings reveal an irony - greater human diversity within an environment leads to less personal diversity.

PS. Thanks to Ian Leslie for the link to the paper.

Image: ragnar1984/Flickr