By Andrea Estrada

For our ancestors, misjudging the physical strength of a would-be opponent might have resulted in painful –– and potentially deadly –– defeat.

Now, a study conducted by a team of UCSB scientists has found that a mechanism exists within the human brain that enables people to determine with uncanny accuracy the fighting ability of men around them by honing in on their upper body strength. What’s more, that assessment can be made even when everything but the men’s faces is obscured from view. A paper highlighting the researchers’ findings appears in the current issue of the Proceedings of the Royal Society.

"Assessing fighting ability was important for our ancestors, and the characteristic that the mind implicitly equates with fighting ability is upper body strength," said Aaron Sell, a postdoctoral fellow at the Center for Evolutionary Psychology and the paper’s lead author. "That’s the component of strength that’s most relevant to premodern combat. The visual assessment of fighting ability is almost perfectly correlated with the perception of strength, and both closely track actual upper body strength. What is a bit spooky is that upper body strength can even be read on a person’s face."

Sell conducted the study with Leda Cosmides, a professor of psychology and co-director of the Center for Evolutionary Psychology; John Tooby, a professor of anthropology and also co-director of the center; and Michael Gurven, an associate professor of anthropology.

The study consisted of four sections, each of which asked the test subjects to assess the physical strength of individuals based on photographs of their faces, their bodies, or both. Subjects were asked to rank the physical strength or fighting ability of the people in the photographs on a scale of one to seven. When the photographs depicted men whose strength had been measured precisely on weight-lifting machines, the researchers found an almost perfect correlation between perceptions of fighting ability and perceptions of strength. "When you see that kind of correlation, it’s telling you you’re measuring the same underlying variable," said Tooby.

They also found that perceptions of strength and fighting ability reflected the target’s actual strength, as measured on weight-lifting machines at the gym. Other sections of the study showed that both men and women accurately judge men’s strength, whether those men are drawn from a general campus population, a hunter-horticulturalist group in Bolivia, or a group of herder-horticulturalists living in the Argentinian Andes.

"Whether people are assessing toughness or strength, it’s upper body strength they implicitly register," said Tooby. "And that’s the critical information our ancestors needed in deciding –– or feeling –– whether to surrender a disputed resource or escalate aggressively."

The ability to judge physical strength and fighting ability serves different purposes for men and women. In men, the mechanism is a barometer for measuring potential threats. For women, it helps identify males who will be able to adequately protect them and their children.