So what is going on?

My own recent research about sisters suggests a more subtle dynamic. I interviewed more than 100 women about their sisters, but if they also had brothers, I asked them to compare. Most said they talked to their sisters more often, at greater length and, yes, about more personal topics. This often meant that they felt closer to their sisters, but not always.

One woman, for example, says she talks for hours by phone to her two brothers as well as her two sisters. But the topics differ. She talks to her sisters about their personal lives; with her brothers she discusses history, geography and books. And, she added, one brother calls her at 5 a.m. as a prank.

A prank? Is this communication? Well, yes — it reminds her that he’s thinking of her. And talking for hours creates and reinforces connections with both brothers and sisters, regardless of what they talk about.

A student in my class recounted a situation that shows how this can work. When their family dog died, the siblings (a brother and three sisters) all called one another. The sisters told one another how much they missed the dog and how terrible they felt. The brother expressed concern for everyone in the family but said nothing about what he himself was feeling.

My student didn’t doubt that her brother felt the same as his sisters; he just didn’t say it directly. And I’ll bet that having the phone conversations served exactly the same purpose for him as the sisters’ calls did for them: providing comfort in the face of their shared loss.

So the key to why having sisters makes people happier — men as well as women — may lie not in the kind of talk they exchange but in the fact of talk. If men, like women, talk more often to their sisters than to their brothers, that could explain why sisters make them happier. The interviews I conducted with women reinforced this insight. Many told me that they don’t talk to their sisters about personal problems, either.