We’ve all heard conversations, comments, and even jokes about how all the members of some race look alike. While that statement is certainly a generalization, it is true that people have a harder time distinguishing between people from a different race than they do within their own race. This phenomenon, called the “other race effect,” was first written about nearly a century ago (and supported by several subsequent studies), but researchers have made little progress on determining why, exactly, this task is so hard for people. Last week, two European psychologists published a paper in PNAS that begins to help us understand the neurophysiological basis of the other race effect.

Twenty-four subjects participated in the study; half were of East Asian descent, the other half were Western Caucasian. Each participant saw a series of two faces presented on a computer screen and had to determine whether the two faces belonged to the same person or not. The two faces were either both East Asian or both Western Caucasian, and were either the faces of two different people, or the same person’s face repeated twice. In all trials, the facial expression changed between faces to make the same face slightly harder to identify.

There are two things you need to know to understand the experiments. In general, when a particular stimulus—such as a face—is shown twice, the neural activity involved in the response decreases the second time; this effect is called repetition suppression. A particular electrophysiological signal in the brain, called the N170 event related potential, is triggered by the sight of a human face.

Due to repetition suppression, if the brain recognizes the second face as being identical to the first, the N170 signal should decrease the second time; if the brain concludes it is a different face, the signal should not decrease in response to the second face.

By subtracting the amplitude of the signal triggered by the second face from that triggered by the first, the researchers could determine how the subjects’ neurophysiological responses differed between the treatments. This measurement was called the single-trial repetition suppression (stRS) value.

As expected, the average stRS value for same race/same face trials was the highest for both East Asian and Western Caucasian subjects. In other words, the neural response decreased the most—signifying that the brain recognized it was seeing the same face twice—when the subjects were presented with a repeated face from their own race.

Interestingly, there was no significant different in stRS values between any other type of trials. For instance, the neural signals that were triggered when a Western Caucasian saw the face of an East Asian twice were no different from the signals triggered when he saw two completely different Western Caucasian faces. The same effect was seen in East Asians; the neural signal showed no recognition when presented with faces from another race.

These results show that N170, the highly-specific facial recognition signal, cannot discriminate between "other race" faces. This inability had not been known previously, and this study is the first to identify a possible neurophysiological basis of the other race effect.

It has been suggested that the other race effect is simply a result of differing amounts of facial variation between races, or varying observational abilities of particular races. However, in this study, subjects of both races showed the same trends, suggesting that the other race effect is a generalized phenomenon experienced by people of more than one race.

PNAS, 2010. DOI: 10.1073/pnas.1005751107 (About DOIs).