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How Strong Is That Guy? The Answer is in His Voice

New research finds humans are equipped to assess the physical strength of a potential adversary by listening to the sound of his voice.
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Henry Wadsworth Longfellow called the human voice “the organ of the soul.” New research suggests it is also a reflection of the body, in that it conveys vital information about the speaker’s physical strength.

That’s the conclusion of a team of researchers led by Aaron Sell, a postdoctoral fellow at the Center for Evolutionary Psychology at the University of California, Santa Barbara. Writing in the Proceedings of the Royal Society B, they report both men and women can accurately assess a man’s upper body strength by simply listening to his voice.

“People are well-designed to assess fighting ability,” Sell told the UCSB press office. “Our previous research shows how the mind uses visual cues to assess fighting ability. But now we have evidence that fighting ability can (also) be detected through the voice. It is part of a complex system of natural selection that helped our ancestors size up their opponents.”

Sell and his colleagues, including John Tooby, Leda Cosmides and Michael Gurven from UC Santa Barbara and Gregory Bryant from the University of California, Los Angeles, obtained voice samples and strength measurements from four distinct populations, including indigenous people from Bolivia and Argentina and college students from the U.S. and Romania.

Groups of UCSB undergraduates were then instructed to rate the person behind the voice in terms of physical strength, height and weight. Some also viewed full-body photographs of the speakers they assessed.

Across the board, when men’s utterances were assessed, “average individual estimates of strength from the voice were accurate and highly significant,” Sell reports. “This accuracy is similar to the accuracy of strength assessment from static visual images of the face, but lower than estimation from images of the body.”

In other words, seeing your adversary’s physique is the best single way to size up his fighting ability, but his voice provides valuable clues as well. The closest estimates of body strength were made by those study participants who both heard a man’s voice and observed his body.

These findings did not hold true when women’s voices were assessed, presumably because physical aggressiveness has, for better or worse, historically been more of a male domain. But they did hold true for males whether the language spoken was English, Spanish, Romanian or the obscure dialect of the Tsimane Indians of South America.

This is the first study to zero in on this evolutionary phenomenon. Sixteen previous studies have examined whether one can judge a person’s height and weight by listening to his voice, coming to no definitive conclusion. Sell believes the inconsistency of those findings can be explained because researchers were measuring the wrong variable.

He points out that, in practical terms — at least in the pre-firearms era — an opponent’s size wasn’t as important as his strength. So it makes sense that humans would develop a shorthand way to determine muscularity rather than height or weight.

“Ancestrally, that’s what the mind needed to know,” said co-author Tooby, “and so that’s what this specialized mechanism hears in the voice.”

So, thank our ancestors the next time you come across a potentially menacing stranger on a dark road. When making a quick fight-or-flight decision, it’s handy to know whether the person in question can beat you up.