The researchers, led by Ken Nakayama and Richard Russell at Harvard and Bradley Duchaine at University College London, have found evidence that prosopagnosia, once thought to be exceedingly rare, may affect up to 2 percent of the population -- suggesting that millions of people may be face-blind.
"Until a few years ago, only 100 cases of prosopagnosia had been documented worldwide, but it now appears the condition is much less rare than had previously been assumed," says Nakayama, the Edgar Pierce Professor of Psychology in Harvard's Faculty of Arts and Sciences. "Testing of 1,600 individuals found that 2 percent of the general public may have face-blindness and a German group has recently made a similar estimate. It's conceivable that millions of people may have symptoms consistent with prosopagnosia, without even realizing it."
Tests developed by Nakayama and colleagues display dozens of images of cars, tools, guns, houses, and landscapes, along with close-cropped black-and-white pictures of faces. Some of the images recur during the cycle; subjects are asked to indicate, as quickly as possible, whether each image they see is new or repeated. Prosopagnosics who take these tests fail to recognize repetition among the faces in the series, even as they readily identify repeated pictures of other objects.
Reports of prosopagnosia date back to antiquity, although it wasn't until 1947 that researchers made the first modern, extensive description of symptoms. The neurological basis of the disorder remains poorly understood, although research has c