The ability to recognise others is a skill we learn early in life, yet even as adults our facility to distinguish between individuals is far from perfect. In particular, researchers have discovered that while we can recognise someone of our own race with relative ease, we find this much more difficult when dealing with individuals of different racial backgrounds. This "own race effect" has been seen equally across a wide variety of races, so just as black people, for example, are better at recognising other blacks, Asian people are particularly attuned to differentiating between people of their own ethnicity.
There is consensus among psychologists that this tendency exists, but deciding why this is the case has proved more problematic. Ian Stephen, a PhD researcher at the school of psychology in the University of St Andrews in Scotland, says one hypothesis is that, in our minds, we hold a prototypical face that represents the average of all the faces we have encountered. If you are white, and grow up in an area with a white majority, then that prototype is likely to be another white face. Similarly, a South Asian person is more likely to hold a South Asian face in their mind as the average.
"You recognise people by how different they are from this and in what way they are different," he says. "It can be envisaged as a space on a page with this prototype in the middle, and other faces are judged by how different they are." Other people's visages come off in different directions and distances from the facial prototype, according to Mr Stephen. Those of a particular race will be clustered together in the same part of the theoretical page, making it harder to tell them apart. Faces of the same race as that individual are placed in different directions and distances from the prototype, and are more easily distinguishable.
A less abstract way of explaining this phenomenon comes from Dr John Brigham, professor emeritus in the department of psychology at Florida State University. One theory, he says, is that people look at the parts of the face that are most diagnostic for distinguishing between individuals of their own race. "So, for example, whites may look at hair colour or eye colour. If the person is black or Asian, there won't be as much variation," he says.
Instead, with Asian people, eye shape is a better variable as it changes more among Asians than among blacks or whites. Support for this idea comes from studies that show people of different races look at different features when reading expressions. In research published last year, Masaki Yuki from Hokkaido University in Japan found American students tended to look at the mouth. By contrast, Japanese people looked at the eyes first when assessing mood. Given that Japanese people, according to the scientists, tend to suppress their emotions, the eyes may reveal more than the mouth.
Not everyone is convinced that there is variation between the races in terms of the facial cues used, at least when it comes to facial recognition. According to Dr Jim Tanaka, a professor in the department of psychology at the University of Victoria, in British Columbia, Canada, people use a "holistic" approach to recognise someone. They do not assess features individually, but rather get a sense of what the face as a whole is like.
"If you see a picture of Brad Pitt, you know it's Brad Pitt, but in recognising him you are not evaluating his nose and mouth individually. It's the emergent whole his features create that gives rise to the impression of Brad Pitt," he says. "My inclination is that we're not really looking at individual features. Eye colour may be diagnostic, but I don't think so. It happens much too quickly. When we recognise a face, we do it in the blink of an eye."
In fact, Dr Tanaka says recognition takes as little as 125 milliseconds. He says people tend to look at the bridge of the nose when they see a person, as this enables them to take in the surrounding parts of the face - the eyes, the nose and the mouth - "in one fell swoop". While we all appear to be better at recognising people of our own race, studies by Dr Tanaka have shown that with training, our ability to recognise those of other races can be improved.
The key, Dr Tanaka has found, is to teach a person to look at the face of someone of another race as an individual, rather than as a member of a race. If this is done, they will better be able to recognise others from that race in the future. "I happen to be Japanese. If you look at me and just categorise me as Japanese, that's a very different visual analysis than if you put an identity to me by name," he says.
The work has been done in Victoria, where Dr Tanaka says there are very few black people or Hispanics. In the study, white university students were trained to recognise individuals of these other races. Over five sessions, they were shown a face of someone of another race and given a name, rather than a racial label, to associate with that face. "They are told: 'This is Joe. This is Bob.' It's simply attaching a proper name or a unique label," he says.
"If you do that you see an improvement in recognition when you show them other faces." The recognition of an individual is accompanied by changes in brain patterns. When electrodes are attached to the scalp, a spike in electrical activity is seen in the part of the brain associated with vision 250 milliseconds after the person is shown the picture of a face that they recognise. "We're already experts at recognising people of our own race, but we're training people to be experts at recognising other races as well," Dr Tanaka says.
"It's not so much focusing on different features, but paying closer attention to those features that define an individual. He or she is perceiving at a finer level of detail. "Certainly races as a group have diagnostic features, but that's only the information we use to categorise a person at the level of race, not as an individual. "Explaining it defies verbal description. How we recognise faces is not something we do in linguistic terms. We do it holistically."