There's appears to be a scientific reason why opposites attract. A study conducted by scientists from the University of Parana in Brazil has shed new light on how humans choose their partners. According to Professor Maria da Graca Bicalho, head of the Immunogenetics and Histocompatibility Laboratory at the University of Parana, people with diverse major histocompatibility complexes (MHCs) were more likely to choose each other as mates than those whose MHCs were similar. In short? Men and women have a tendency to choose a mate with a different DNA make-up than their own.

The study involved MHC data from 90 married couples. The data was compared against 152 randomly generated control couples. The researchers counted the number of MHC dissimilarities among those who were "real" couples, then compared them with those in the randomly-generated virtual couples. "If MHC genes did not influence mate selection, we would have expected to see similar results from both sets of couples. But we found that the real partners had significantly more MHC dissimilarities than we could have expected to find simply by chance," says Bicalho, who presented the findings at a conference of the European Society of Human Genetics in Vienna this past Monday.

The scientists believe that the MHC effect may be an evolutionary strategy that helps prevent incest and improves immune system efficiency in order to ensure healthy reproduction. "Although it may be tempting to think that humans choose their partners because of their similarities, our research has shown clearly that it is differences that make for successful reproduction, and that the subconscious drive to have healthy children is important when choosing a mate," says Bicalho. In a summary prepared for the meeting, the researchers wrote: "Parents with dissimilar (genetic regions) could provide their offspring with a better chance to ward infections off because their immune system genes are more diverse."

How exactly do humans know who might be genetically different? While scientists are not clear on which signals make us attracted to people who are genetically different from ourselves, they believe body odor and face structure may play a role. In fact, past studies have shown that animals may use body odor as a guide to identifying genetically different mates, although Bicalho emphasizes that other factors may be involved. "Other cues such as face symmetry might play a role as well, but they are still in the field of speculation," she says.

News Release: Opposites attract in human search for mate www.reuters.com May 24, 2009