Scientists at the University of California, Santa Barbara has found that people can determine with uncanny accuracy the fighting ability of men around them by honing in on their upper body strength, by just looking at their faces.
"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 UCSB's Center for Evolutionary Psychology and the paper's lead author.
He added: "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.
The UC study comprised 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.
The participants in the study were asked to rank the physical strength or fighting ability of the people in the photographs on a scale of one to seven.
Almost a perfect correlation between perceptions of fighting ability and perceptions of strength was obtained when the photographs depicted men whose strength had been measured precisely on weight-lifting machines.
"When you see that kind of correlation it's telling you you're measuring the same underlying variable," said one of the co-authors of the study.
The researchers also observed that perceptions of strength and fighting ability reflected the target's actual strength, as measured on weight-lifting machines at the gym.
The study also found that both men and women accurately judge men's strength, no matter which place or occupation they belonged to.
The researchers also measured leg strength in some populations, but the results showed that perceptions of men's strength and fighting ability reflect upper body strength, not that of legs.
"The next step is to isolate what it is in the face that indicates upper body strength," said Sell.
He speculated that the correlation might lie in the heavier brow ridge and thicker jaw that result from increased levels of testosterone.
"Many studies have been done on the effects of testosterone on the face. There's a good chance testosterone is involved in regulating the body for battle, and men with high testosterone - those with a heavy brow ridge and thicker jaw - developed bodies that were more prepared for combat," said Sell.
"One reason we evolved the ability to perceive physical strength in the face may be that it's where we focus our attention when we look at someone. Even if we are able to see someone's body, we always look at the face. It's so rich in social information - what a person is thinking or feeling - and adding the assessment of physical strength is a huge benefit. A person who is angry and strong offers a much greater threat than the person who is angry but weak," said another co-author of the study.
A paper highlighting the researchers' findings appears in the current issue of the Proceedings of the Royal Society.