Do you have trouble picking out a friend’s face among a group of people? There’s a name for your condition: prosopagnosia, or face blindness. The disorder was thought to be exceedingly rare and mainly a result of brain injury. But last month a team of German researchers took the first stab at charting its prevalence, and the results were remarkable. The new study showed that prosopagnosia is highly heritable and surprisingly common, afflicting, in some form, about 1 in 50 people—more than 5 million in the US alone. “That’s huge,” says Dr. Thomas Grüter of the Institute of Human Genetics in Münster. “It was a real surprise.” Within that group of sufferers, however, the condition varies widely. For the vast majority, the problem is not so much about detecting a face—prosopagnosics can see eyes, noses, and mouths as clearly as anyone else—as it is about recognizing the same set of features when seeing them again. While mild prosopagnosics can train themselves to memorize a limited number of faces, others grapple with identifying family members and, in extreme cases, their own faces. Gaylen Howard, 40, a homemaker in Boulder, Colo., says that when she’s standing in front of a mirror in a crowded restroom, she makes a funny face so that, as she puts it, “I can tell which one is me.” Most prosopagnosics learn to cope early on. They distinguish people based on cues like hairstyle, voice, or body shape. They shun places where they could unexpectedly run in..