Men's deep, resonant voices did not evolve to thrill the ladies, but rather are likely to scare off their rivals, researchers say.
A study of how people respond to voices shows that men react far more strongly to a deeply pitched male voice than women do.
And humans, it seems, have the biggest sex-based differences in voice pitch of any primate. Women's voices are far higher and men's far lower, the team at Penn State University found.
"A lower pitch made men attractive to women. But it especially made men seem more dominant to other men," said anthropologist David Puts at Penn State, who led the study team.
Puts studies gender-based differences and what they mean. In other primates, the differences between males and females tend to become more obvious when there’s a lot of competition for mates, he said.
They collected thousands of recordings of primates from around the world, from monkeys to apes. They found the difference in voice pitch correlated with mating patterns.
"When a species evolves towards more polygyny - more male competition for females -- the species tends to evolve more sex differences," he said. "With monogamy, the sex differences diminish."
Anthropologists already know that with humans, the size differences are not as big as with gorillas and orangutans, where males are much bigger than females. And female humans don't always choose their mates in easily predictable ways.
"We find that masculine traits in humans are not the same as, say, in peacocks where the beautiful tail attracts a mate," said Puts. “For example, beards make men more dominant-looking, scarier and seemingly more dangerous, but most women prefer clean-shaven men."
And the voice differences are large, too. Men's voices tend to come in at around 110 hertz (Hz), while women's are much higher, at 200-220 Hz.
"We are trying to understand the evolution of these really large sex differences in voice pitch that we see in humans and some other primates as well," Puts said.
They recorded more than 500 men and women speaking and then played the recordings back to more than 1,100 different men and women.
Each female recording was rated for attractiveness by 15 men. Each male recording was rated by 15 men for dominance and 15 women for short- and long-term romantic attractiveness.
The lower the voice, the more other men thought the voice belonged to a dominant man, Puts and colleagues reported in the Proceedings of the Royal Society B.
To a lesser degree, women liked lower-pitched voices too, but it was not consistent.
The study also suggests why women manage fine whether they have high voices or a throaty purr. Men didn't seem to care much about women's voices.
"We looked at female pitch and we just didn't find anything," Puts said. Men liked high-pitched and low-pitched women’s voices just fine.
Then they tested some men and women for cortisol, a stress hormone, and testosterone, the so-called male hormone. Consistently higher cortisol levels tend to damage health.
Men with low-pitched voices had lower levels of cortisol and higher levels of testosterone, they found.
"It tells something about their condition, their health, their formidability," Puts said.
"In humans and possibly other primates, too, a low pitched voice evolved primarily to intimidate other men … to make males seem bigger and scarier."