If I want you to give time or money to my cause, I'll say your sacrifice is for "people just like you, just like me," for "communities like yours, all across America," or, as Mary Tyler Moore once wrote, for "fascinating beings with complex social interactions, long childhoods and awkward adolescences" (her subject was lobsters—so human-like, she said, they even walk "claw-in-claw" on the seafloor). Feelings of kinship promote feelings of kindness, as marketers know. Last week, this paper in Science suggested one possible reason.

Dosed with the "cuddle chemical" oxytocin, men in the experiments became more generous to total strangers who were members of the same team. Other men, who got a placebo up their noses instead of the oxytocin spritz, were far less likely to help a fellow "Triangle" or "Circle." (Importantly, the teams were what social psychologists call "minimal groups," created right before the experiment began. It is incredibly easy to get human beings to share an identity. That's why those appeals to help "people like you" are so effective.)

Oxytocin has gotten a lot of "cuddle chemical" press lately, because it's associated with warm, touchy-feely emotions in one-on-one situations. Levels of oxytocin rise after orgasm and during childbirth and breast-feeding, for instance, and go up in children when they are comforted by their mothers. In one recent experiment, men given oxytocin became more sensitive to social cues and more empathetic—almost as empathetic, in fact, as women, said the experimenters. The "bonding hormone" has even been tried as a means of getting autistic people to be more alert to other people's feelings.

The new Science paper, though, is the first I've seen to look at how oxytocin might interact with perceptions of groups instead of just individuals. Among the things I liked about it was its pushback against psychology's individualist orientation.

After all, in the course of a typical day you encounter a lot of people whom you never perceive as individuals. Rather you'll see them as "a cop," "a mother," "a guy driving a Mercedes," "a New Yorker," and so on. And your response to each will vary depending on the identity you perceive. Yet many theories describe the male brain or the child's development or the use of stereotypes, as if all people mean the same thing to one another, and always have the same interaction with each new person they engage.

Well, as your mother the New York cop who drives a Benz might say, Forget about that. Individualist psychology would predict that oxytocin, the "bonding hormone," requires sex, hugs, intimate conversation or some other one-on-one interaction. But these new experiments, performed by Carsten De Dreu and his colleagues at the University of Amsterdam, show that the mere idea of shared membership in a group is enough. And unlike individualist accounts of oxytocin, which imply that it will have the same effect on any two people, De Dreu's work found that oxytocin's cuddly influence stops at the border between "Us" and "Them."