In academic circles at least, women tend to cooperate with same-sex individuals of higher or lower rank less often than men do. So say researchers who report evidence in the Cell Press journal Current Biology. The findings are based on a study of the publication records of professors working at 50 North American universities.

"People are often upset to hear evidence of sex differences in behavior," says Joyce Benenson of Harvard University. "But the more we know, the more easily we can promote a fair society."

The findings might seem somewhat counterintuitive. People often expect that women are more cooperative than men. But then again, in humans, as in our chimpanzee ancestors, it is males that more frequently band together with other males to support one another in rivalries against other groups. Think of informal street gangs or armies.

"The explicit hierarchies that form within a male community based on overt competition therefore must be continually negotiated so that individuals who end up higher or lower ranked nevertheless feel strong enough ties to their group that they are willing to cooperate to defeat another group," Benenson explains. "This tendency in human males extends to groups formed for reasons other than fighting." Females, on the other hand, may be more likely to interact with smaller groups or even a single individual.

To explore these dynamics in our modern world, Benenson and her colleagues looked to academia. Actually, they didn't start there, but they found rather quickly that the basic information they needed - individual rank, evidence of mutual investment, and a baseline number of males and females - couldn't be found in the military, government, or business. The number of women professors was unfortunately also too low in disciplines including biology, chemistry, and physics. It was within psychology departments that the researchers finally found the data they were looking for.

Using numbers of coauthored peer-reviewed publications as an objective measure of cooperation and professorial status as a measure of rank, the researchers calculated the likelihood of coauthorship with respect to the number of available professors in the same department. Their calculations showed no difference between men and women at all among individuals of equal rank. But male full professors were much more likely than female full professors to coauthor publications with a same-gender assistant professor. In other words, differences in rank didn't get in the way of cooperation amongst men in the way that they apparently did for women.

Benenson says the researchers are planning a number of follow-up studies to answer lingering questions: Do men somehow prevent women from cooperating more? Do women attempt cooperation across rank and fail for one reason or another? Is the reluctance to cooperate driven from above or below? And what would encourage women to reach out and cooperate more?

The bottom line for now is this: "In ordinary life we often think of women as being more cooperative and friendly with each other than men are, but this is not true when hierarchy enters the picture," Benenson says.