Head-turner. Men with more symmetrical bodies and broader shoulders (left) score higher on the attractiveness scale.

William Brown/ Brunel University

Oh, Brad, You're So . . . Symmetrical

Ask a woman if she thinks actor Brad Pitt's body is attractive, and the answer will most likely be a resounding yes. But she might be hard-pressed to explain why that heartthrob excels over the merely buff. Researchers can now provide some help. Pretty people such as Pitt tend to have more symmetrical bodies, scientists have found. But questions remain about whether symmetry reflects any kind of evolutionary advantage.

With two hands, two legs, and two sides of the brain, humans are clearly bilateral beings. But bilateral doesn't mean perfectly symmetrical: One earlobe might hang slightly lower than the other, someone's left wrist might be just a bit wider than their right. Studies of faces have shown that people perceive more symmetrical faces of either gender as more attractive, perhaps because these subtle differences, called fluctuating asymmetry, somehow reflect susceptibility to environmental stresses such as disease. Vulnerability to stress might make someone less healthy and therefore less suitable as a mate, according to the theory. Isolating asymmetry in the rest of the body has proven difficult because researchers lacked instruments designed to measure these differences.

William Brown, an evolutionary psychologist at Brunel University in the U.K., decided to take the problem to people who know bodies best: doctors and fashion designers. Physicians sometimes use three-dimensional body scanners to make images of a burn victim's skin, and designers use them to make more accurate models for fitting clothing. Brown's team used the device to scan the bodies of 40 male and 37 female college students. A computer gave precise measurements for 24 traits: ankle girth, shoulder width, and others. The computer also created a full-scale, rotating image of each subject's body. The model displayed no information about skin color, clothing, or hairstyle. The researchers then asked 37 men and 50 women to rate attractiveness of the opposite-sex models.

The more symmetrical bodies in both sexes were judged as more attractive, the researchers report today in the Proceedings of the National Academy of Sciences. Brown speculates that this balance could reflect fewer stresses during development. "What Darwin would have called secondary sex characteristics"--hip and chest dimensions, for example--"may be indicating better development," Brown says.

Evolutionary biologist Joel McGlothlin of the University of Virginia, Charlottesville, applauds the use of digital imaging to measure fluctuating asymmetry. But researchers don't have enough evidence to say that this asymmetry reflects problems that occurred when a child was growing up, he notes. It could just reflect a person's health at the time or other factors. And, more symmetrical shoulders might come simply from lifting weights.