Ever hear a woman say, “He seems perfect, but there’s just no chemistry”?

Well there may be something to her assessment, says Kansas University psychology professor Steve Ilardi.

There’s a set of genes called the Major Histocompatibility Complex (MHC) that plays a role in programming the human immune response. It’s been found that two people with very similar MHC sequences who mate are likely to have children with much less effective immune systems, Ilardi says.

Studies have shown that women – who have a highly developed sense of smell compared to men – can actually sniff out MHC compatibility in a potential mate.

In the experiments, men were asked to wear a T-shirt for an extended length of time without bathing. The garments were then sealed in a plastic bag and given to women, who rated how attracted they were to each scent. Overall, women were much more drawn to the “aromas” of men with different MHCs than their own, thus increasing the likelihood that they would have healthy children.

“So sometimes when you’re brain is telling you, ‘I should fall for this guy; he seems perfect, but there’s no chemistry’ – that could be because your MHCs are too close,” Ilardi says. “It’s almost like when a woman says, ‘It’s like kissing my brother.'”

Scientists also know that humans are more attracted to people who have symmetrical features and bodies, which generally equates to better health and higher IQ.

The same sort of scent studies have shown that women also can sniff out symmetry in a potential mate.

“It’s really disturbing and gross and horrible, but it’s fun to talk about,” Ilardi says. “When I introduce this to students, they have a lot of fun with it.”