Compact face: The study shows that adult males have relatively shorter upper faces. In comparison to the female face, the male face is wider and the upper facial height is approximately the same.

Credit: PLoS ONE / Weston et al.

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PARIS: Forget jutting jaws, pheromones or hypnotic stares. One factor making men sexy since prehistory could well be a foreshortened face, according to a new study.

Theories abound as to why humans are attracted to one another, and on the role facial features might have played in the human evolutionary saga.

But palaeontologists at the Natural History Museum in London, England, have now uncovered something that has so far gone unnoticed: for at least the last two million years, the space between the brow and upper lip in hominids has been, proportionately, shorter and wider in males than in females.

Mate choice

"There has to be a reason why, at puberty, the face of men and women develop differently," said lead author Eleanor Weston, pointing out that there is no plausible mechanical explanation for the divergence. Which leaves sex.

"There is evidence to suggest that sexual selection – operating mainly through mate choice, has shaped the human face," she said.

Over the course of evolution, Weston argues, females may have been drawn to males whose smaller middle face accentuated all the peripheral trimmings: bushy eyebrows, strong cheekbones, squared jaw.

A man with a compact face, in other words, is a turn on.

Weston, who describes her work as "pioneering," said that her hypothesis on human evolution has yet to be rigorously tested, and could prove controversial.

She also acknowledged that – even if upper face size may turn out to have be a critical element in the sex life of early hominids – the same facial features may have lost some of their magnetic charm today.

"But I do still think that, biologically speaking, this represents masculinity is some way," and confers a competitive advantage, she said.

"Hidden characteristic"

Weston hopes that her study, published in the journal PLoS ONE, will inspire colleagues, especially psychologists, to design experiments to test her theories about attractiveness. In the meantime, she has done an informal analysis of males faces widely considered to be handsome.

"If you measure this ratio in film stars – generally thought to be very attractive – the [ratio is] much lower," she said, hastening to add that this was not part of her published study. "It is like a hidden characteristic that people haven't actually tested in their simulations of masculinity."

Beyond the realm of speculation, the findings of Weston and her colleagues could help identify the sex of early hominid fossils, a notoriously difficult task. The few faces of fossil hominids that are preserved, she pointed out, plot exactly on the male-female modern growth trajectory she has uncovered.

But "it will take quite a big leap of faith for anthropologists to get rid of the idea of calling it a male just because it is bigger, and looking instead at the facial proportions," she predicted.