Helen Fisher, an anthropologist at Rutgers University in New Jersey, said for men the kisses appear to provide data on a woman's estrogen level, The Daily Mail reported. That would let them know where the woman is in the fertility cycle.
"Men like sloppier kisses," she said.
Women, on the other hand, may be getting unconscious information on whether a potential partner has an immune system different from theirs. Marrying someone with complementary immunity could lead to healthier offspring with a broad range of disease resistance.
Fisher reported on her research at the annual conference of the American Association for the Advancement of Science in Chicago.
"I think that is just the beginning of what we are going to find out," she said. "This is just the tip of the iceberg. We are going to find many other mechanisms we unconsciously use to size up a person's biological traits."