It's long been known that an attractive face is highly symmetric - but now, Anthony Little of the University of Stirling, Scotland, and colleagues have gone one step further: they are attractive because they advertise genetic quality or fertility.
Using mug shots of Europeans, the Hadza of Tanzania, one of the last hunter gatherer cultures, and macaque monkeys, measurements were made and people were asked to judge the masculinity of the most and least symmetric pictures.
Whether a member of a troop or a tribe, symmetric males had more masculine facial proportions and symmetric females had more feminine facial proportions.
"In humans, if you look at female models, for example, they tend to be pretty symmetric and at the extremes of femininity," Dr Little says.
He adds "One good face trait deserves another - symmetric men and women appear to have other good face traits".
The findings back the claim that the masculinity/femininity of faces is linked with symmetry and hence advertise quality, that is good genes.
Biological quality can mean many things but as symmetry and femininity/masculinity arise during development then one explanation for the findings is that "both traits could advertise quality in terms of resistance to disease, or environmental stresses and that might mean people with these traits are healthier and live longer," says Dr Little.