Australian researchers have discovered the genes that lie behind the attraction quotient in men.
After conducting tests on more than 70 Aussie blokes, University of Western Australia researchers found that women like the look of men with a lot of diversity in their immune system genes.
The men in the study were judged to have more average facial features - not too big or not too small - a characteristic humans find appealing.
In fact, our preferences in facial features have evolved as a way of picking a high quality mate. The women's preferences remain remarkably similar across all cultures.
Men with genetically diverse immune system genes were likely to be healthier.
"Genetic diversity is a form of genetic quality - the opposite of being inbred," Stuff.co.nz quoted her as saying.
She added that the genetic quality was reflected in their faces. The facial features that people around the world find most attractive are symmetry, averageness, femininity in women and masculinity in men.
"Men are swayed a lot by femininity," said Lie.
The researchers tested the genetic diversity of 77 men and 77 women, looking right across their DNA code, as well as at a cluster of immune system genes, known as the major histocompatibility complex, which helps fight off infections.
They used standardised photographs, and then rated the people by the opposite sex for their facial attractiveness, symmetry, averageness, and femininity or masculinity.
The diversity of a man's immune system genes predicted his appeal to women, while there was no link between his overall genetic diversity and his facial attractiveness.
However, women who had greater overall genetic diversity had more symmetrical faces.
Lie said that as women invested a lot in child-bearing and raising, thus mate choice was particularly important to them. They speculated that it may be because they were particularly good at picking men's genetic quality from facial clues.
"From an evolutionary point of view, women should be more choosy than men," she said.
Also, the researchers found that men with greater diversity in their immune system genes tended to have healthier skin, which may indicate their genetic quality apart from average features. Women also prefer their smell.
However, she said that genes were not everything. Women choose partners on many factors, such as proximity in age and social background.
"And men can do a lot of things to offset their lack of attractiveness, like dress well," she said.
The results are published in the journal Evolution.