In the 1950s, sociologists coined the term “homophily” — love of the same — to explain our inexorable tendency to link up with one another in ways that confirm rather than test our core beliefs. Those who liked Ike, in other words, liked each other. The term didn’t catch on, but the concept is now enjoying a renaissance, in part because it has been repeatedly invoked to explain the American electorate’s apparent polarization into equally self-regarding camps.

“Similarity breeds connection,” the sociologists Miller McPherson, Lynn Smith-Lovin and James Cook wrote in their classic 2001 paper on the subject, “Birds of a Feather: Homophily in Social Networks,” and “the result is that people’s personal networks are homogeneous.” This year, other academics have cited homophily in elucidating everything from why teenagers choose friends who smoke and drink the same amount that they do to “the strong isolation of lower-class blacks from the interracial- marriage market.” Researchers at M.I.T. even published “Homophily in Online Dating: When Do You Like Someone Like Yourself?” which showed that you like someone like yourself most of the time, online or off. So much for “opposites attract.”

The Web site O’Reilly Radar, in its continuing discussion of technology trends, raised the question earlier this year of how to get opposites back into the equation. In “Homophily in Social Software,” Nat Torkington, a trend spotter for O’Reilly Media, argued that “homophily raises the question for social-software designers of how much they should encourage homophily and how much they want to mix it up.” Social-software designers are the people behind Web sites like Facebook and MySpace, which tend to bring birds of a feather together. Meanwhile, chains of recommendations (“if you liked . . . ”) on sites like Amazon reinforce our original preferences even as they claim to expand our horizons.

To counter the seemingly universal trend toward homophily, Torkington invited his readers to figure out how to create “serendipity.” One information-technology specialist described a feature he would add to Facebook called “the Stretch,” which would help students “find a group of people a little different” from themselves. Someone else brought up the online book cataloger LibraryThing’s UnSuggester, which identifies the book least likely to share a library with the book you mention.