Have you ever wondered why you seem to understand some people—even if you know relatively little about them? It turns out there may be a biological reason why it's easier to walk a mile in some people's shoes but not in others'. Researchers report in Proceedings of the National Academy of Sciences USA that brain scans suggest people project their own values and feelings onto others if there is even the slightest evidence that the pair have something in common.

Scientists from Harvard University and the University of Aberdeen in Scotland say the reason is that nerve cells, which fire during self-evaluation, also swing into action when people are asked to predict how another person might feel—if, that is, they believe the person would act similarly to them. If, however, they are convinced their peers are not of a same mind, so to speak, those neurons remain inactive.

They say the new finding paves the way for research on how stereotypes may grow from the tiniest seeds into major misconceptions.

"It might help explain why people who learn some small piece of information about some other person," says study co-author Adrianna Jenkins, a psychology graduate student at Harvard, "may have difficulty communicating with that person on a variety of topics."

In this study, 13 volunteers were asked to draw conclusions about two characters based on short sketches containing only basic tidbits about them: One character was portrayed as a liberal Northeasterner, who attended a small, private college; the other was depicted as a conservative Midwesterner, who attended a large university.

The participants disclosed with whom of the two characters they had a closer identification and were asked a series of questions about that person—and themselves—as their brains were scanned by functional magnetic resonance imaging (fMRI). Researchers focused their examination on a small population of neurons in the ventromedial prefrontal cortex (vmPFC), which is known to be involved in self-referential thought as well as emotion. Among the queries: whether they enjoy being the center of attention, like impressionist art or if they would readily help a friend in need. They were also asked to predict how the characters in the sketches would respond.

Cells in the targeted brain regions fired when researchers asked subjects about their own views; they would also activate if such questions were followed by queries about the character with whom the volunteer most identified. There was not, however, any activity when subjects were queried about the dissimilar characters. The results, "certainly suggest that for people we perceive to be similar to us," Jenkins says, "we are sort of automatically bestowing upon them the rich complement of [our own] characteristics…."

Jenkins says that she would now like to study brain activity that occurs when judging people dissimilar to ourselves. Stephen Macknik, a neurophysiologist at Barrow Neurological Institute in Phoenix, says such research could help determine the neural basis for prejudices and exclusionary behavior.

"You could [also] imagine a lot of evil uses for this," he cautions, noting that it could fit in with recent talk about using an fMRI as a lie detector. "You could use this type of technology to determine how someone really feels about someone else, even if they say something different."