“That’s one of the things that happens when you become a couple: you date other couples.” --Nora Ephron

I don’t know if we really needed a study to confirm what Nora Ephron observed long ago, but now we have one. Research shows that couples befriend other couples.

The researchers followed 126 people, all coupled, over a 9-month period, asking them repeatedly about their friends. The participants all lived in graduate student housing set aside specifically for married couples and couples in long-term cohabiting relationships. (Whether universities should give special housing preference to couples is an issue worth discussing, but I won’t do that here.)

Over time, the couples tended to pair up with other couples far more often than would happen just by chance. Describing how she and her husband related to two other couples, Nora Ephron said, “We saw each other every Saturday night and every Sunday night, and we had a standing engagement for New Year’s Eve.” The researchers did not elicit that level of detail from their participants, but they did find that couples befriending other couples was a thing.

Within the pairs of couples, sometimes the two women were friends with each other and the two men were friends with each other. (All but two of the couples were heterosexual.) But the reverse was almost never true: the man from one couple was not friends with the woman from the other and vice versa.

I first started reading the study because I thought it was going to address a question I’ve been interested in for a long time. We know from other research that couples become more insular when they move in together or get married. They see their friends less often (and also have less contact with their parents). But are they especially likely to ditch their single friends? I still don’t have a definitive answer, because it turned out that in the study, all the participants were coupled, and they were only asked about their friends within their university housing community—and all those people were coupled, too.

I almost stopped reading the article when I realized that, but I’m glad I didn’t. I realized from reading more of the findings that the couples seemed stuck together in ways that were constraining. Suppose, for example, that Mary and Martin were a couple, and early on, Mary was also friends with Nancy. Does she stay friends with Nancy? Probably not, unless Martin becomes friends with her, too. If Mary is going to stay friends with Nancy, probably what will happen is that Mary and Martin, as a couple, will become friends with the couple consisting of Nancy and Norbert.

Apparently, in the world of couples, three is an unwelcome number. If Mary is friends with Nancy, then either Martin will become friends with her, too, or Nancy will get ditched. Similarly, Mary is not likely to stay friends only with Nancy; she will also end up friends with Norbert—or, again, Nancy will get ditched. Ultimately, the two couples either start each other or they go their separate ways.

Mary’s prospects for an enduring are especially dire if her friend is a guy. If Mary wants to stay friends with Tom, then she had better get Martin to strike up his own friendship with Tom. The authors describe this, unselfconsciously, as a “control mechanism.” Martin is controlling Mary’s friendships to preserve his romantic tie with Mary. That does sound romantic, doesn’t it? (The reverse happens, too—Mary doesn’t want Martin hanging out with Cheryl, though maybe she will come around if she cultivates her own friendship with Cheryl.)

The authors are so sure that romantic partnerships are just better than platonic friendships (they repeatedly call them “more valuable”) that they never consider another possibility: may constrain friendship choices, with the result that the friendships of coupled people do not, on the average, measure up to the friendships of single people.

That’s just a guess. But if single people have more freedom to choose as friends the people they really click with, whereas coupled people are more likely to be boxed in by their partner’s preferences, then well…maybe the friendships of single people are going to be more enduring, and deeper, too.