People can gauge aggressiveness in their evolutionary cousins' expressionless faces.
People can read chimpanzee's faces and tell if the animal is dominant and physically active.
The ability to discern key personality traits through faces evolved more than 7 million years ago in our shared ancestors.
In chimpanzees, as in humans, faces are personality billboards, a new study suggests.
People can usually tell whether or not a chimp acts dominantly and is physically active simply by looking at a picture of the ape's expressionless mug, says a research team led by psychologist Robert Ward of Bangor University, Wales.
Consistent with earlier evidence from other researchers, Ward and his colleagues reported last year that volunteers can also accurately detect whether people are extroverted, emotionally stable, agreeable and imaginative by looking at pictures of their neutral-looking faces. Extroversion in people and dominance in chimps both relate to assertiveness and sociability, and both partly derive from an individual's genetic makeup.
An ability to discern key personality traits via facial structure evolved more than 7 million years ago in a shared ancestor of people and chimps, the researchers propose in a paper published online Jan. 14 in Evolution and Human Behavior.
"The fact that chimpanzee facial signals can be read by humans suggests that our ability to read others' faces accurately is not solely acquired through culture, but is part of an evolved system," Ward says.
That's an intriguing hypothesis in need of testing with composite images that digitally combine many pictures of the same chimps into single mug shots, remarks psychologist and chimp researcher Lisa Parr of Emory University in Atlanta. Composites minimize slight variations from one photograph to another in lighting, skin hue, head angles and other factors that can create different personality impressions of the same individual, Parr says.
Ward and his colleagues had participants evaluate composite images of people, but technical difficulties stymied their attempts to create composite chimp faces. Anatomical landmarks used to create composite images, such as the jaw's precise position, are difficult to measure on chimps' hairy faces, Ward says. Also, composites smooth out facial textures, so chimps' faces look blurry rather than hairy, he notes.
In the new study, Ward's group conducted four experiments with a total of 139 college students. Volunteers viewed pairs of mug shots showing chimps previously identified in behavioral observations as high or low in dominance. Each photographed chimp looked straight ahead or at a slight angle, with no teeth showing and no strong shadowing over the eyes that might impart a menacing look.
Participants distinguished dominant from nondominant chimps more often than would have been expected by chance. Average accuracy rates ranged from about 60 percent to 70 percent, with higher scores for faces of male chimps than for female chimps.
The students accurately distinguished mug shots of extroverted women from those of introverted women about three-quarters of the time. Pairs of pictures came from women who reported many similar personality traits on a questionnaire, except for contrasting levels of extroversion.
Core facial characteristics of dominant chimps and extroverted people remain poorly understood, Ward says. A well-defined jawline and other outer features of the face aid in detecting extroverted women, but volunteers still do pretty well at distinguishing outgoing from shy females when shown only each person's eyes, nose and mouth.
Ward suspects that chimps who look at other chimps' expressionless faces can tell which ones act dominantly. His team plans to explore this possibility.