What's in a face? A chimpanzee's face, that is. Humans, it seems, apparently remain genetically and culturally close enough to their primate relatives, and are able to accurately guess an individual chimp's personality just by observing it s face. This discovery sheds light on the evolution of our faces, and could yield insights into the nature of autism.

There's an old saying that you can't judge a book by its cover, meaning that one shouldn't judge someone or something based solely on appearances. Recent findings suggest, however, that one actually might be able to correctly guess many aspects of another person's personality just by looking at his or her face if it displays a neutral expression, including traits such as trustworthiness and aggression.

Our faces could be windows into our souls because our ancestors developed them as ways to honestly tell others about ourselves—knowledge that could help social interactions proceed more smoothly. If our brains, faces and ability to analyze them did evolve in this manner, social psychologist Robert Ward at Bangor University in Wales and his colleagues reasoned that we might share part of this system with chimpanzees, humanity's closest living relatives.

In four experiments scientists asked university students to look at chimp faces. First, 43 volunteers viewed photos of chimpanzee faces bearing neutral expressions— for instance, they were not baring their teeth.

The researchers had previously assessed the 37 apes in these pictures according to 15 personality factors, such as "dominant," "active," "sociable" and "sympathetic." Volunteers were shown two photos at a time, each of a different chimpanzee, and asked to guess which one best fit a certain personality trait than the other. People typically accurately identified "dominant" and "active" personalities correctly, and nearly correctly guessed "sympathetic" ones to a significant degree. Intriguingly, volunteers often seemed to judge "sociable" incorrectly, suggesting they misinterpreted what they saw.

In a second experiment Ward and his collaborators had 30 different students rate the chimp faces on a seven-point scale on the four traits identified in the first experiment — dominant, active, sociable and sympathetic. After comparing these results with the personality ratings scientists previously had of the chimpanzees, the researchers found a significant number of the volunteers were able to accurately guess how dominant the apes were.

A third experiment supported these results — 30 more students were shown pairs of chimp faces, each of a different chimpanzee of the same sex, and they typically correctly identified the more dominant ape, proving more accurate with male chimps than females. Dominance, as the scientists described it to volunteers, involved being more able to displace, threaten or take food from others —or more likely to show their high status by decisively intervening in social interactions.

In the last experiment the scientists investigated how well volunteers were able to assess both the dominance of chimps as well as how extroverted a human might be. (In humans psychologists typically consider dominance an extroverted or outgoing personality trait.)

The researchers first had another 36 students complete tests measuring personality, empathy and autism. The volunteers then compared pairs of chimp faces for dominance, and also looked at pairs of black-and-white photos of white women's faces that registered neutral expressions , each of whom scientists had assessed for extroversion.

As before, volunteers typically judged chimpanzee dominance accurately, and also often guessed female extroversion correctly. Interestingly, volunteers whose social skills embodied autisticlike traits were worse at reading the signs of extroversion in the women but did fine when judging chimps. "We are thinking this might reflect additional human-specific social cognition," Ward says. "That is, there would be a set of social skills and traits that we share with our evolutionary relatives, and another set specific to humans and our own adaptive problems."

Overall, these findings suggest the way our faces are used to send messages dates back at least to the last common ancestor of humans and chimpanzees. "It isn't just cultural training that allows us to predict behavior in this way," Ward says. "The cross-species case suggests there can be some underlying biology in the signal system linking our facial appearances, our behaviors, and our perceptions of others."

Future research can investigate which details in the face actually signal personality traits such as dominance. Also, there are developmental and congenital syndromes for which facial appearance is linked with mental issues, such as Williams syndrome, "in which the so-called 'elfin' facial appearance is also associated with extremely outgoing natures," Ward says. "We would like to know more about the factors that influence both facial development and behavior."

"It would be really interesting to know how chimpanzees would use these cues rather than humans," says primatologist Bill Hopkins at Emory University, who did not take part in this study. "Obviously, this is more difficult to do experimentally but it would definitely be worth pursuing."

In addition, "it would be interesting to take photos of young apes and ask whether one can predict their personality as an adult," Hopkins suggests. "It would also be interesting to see whether the subjective rating of personality based on the photos would predict individual differences in some behavioral tasks, such as approach-avoidance or food-sharing."

Ward, along with Robin Kramer and James King, detailed their findings online January 14 in Evolution & Human Behavior.