Opposites attract and genetics influence why you choose your partner

Brazilian scientists say they have discovered how humans choose their partners - they say opposites attract and genetics influence how a person chooses a partner.

According to Professor Maria da Graça Bicalho, head of the Immunogenetics and Histocompatibility Laboratory at the University of Parana, Brazil, people with diverse Major Histocompatibility Complexes (MHCs) were more likely to choose each other as mates than those whose MHCs were similar, and that this was likely to be an evolutionary strategy to ensure healthy reproduction.

The Brazilian team has been working in this field since 1998 and they say a females' preference for MHC dissimilar mates has been shown in many vertebrate species, including humans, and it is also known that MHC influences mating selection by preferences for particular body odours.

The researchers decided to investigate mate selection in the Brazilian population, while trying to uncover the biological significance of MHC diversity - this involved studying the MHC data from 90 married couples, and then comparing them with 152 randomly-generated control couples - they counted the number of MHC dissimilarities among those who were real couples and compared them with those in the randomly-generated 'virtual couples'.

Professor Bicalho says if MHC genes did not influence mate selection, similar results would have been expected from both sets of couples - it was in fact found that the real partners had significantly more MHC dissimilarities than could have been expected simply by chance.

The scientists say within MHC-dissimilar couples the partners will be genetically different, and such a pattern of mate choice decreases the danger of endogamy (mating among relatives) and increases the genetic variability of offspring.

They say genetic variability is known to be an advantage for offspring, and the MHC effect could be an evolutionary strategy underlying incest avoidance in humans and also improving the efficiency of the immune system.

The researchers say the MHC is a large and extraordinarily diverse genetic region situated on chromosome 6, and found in most vertebrates and it plays an important role in the immune system and also in reproductive success.

Professor Bicalho says although it may be tempting to think that humans choose their partners because of their similarities, their research has clearly shown that it is differences that make for successful reproduction, and that the subconscious drive to have healthy children is important when choosing a mate.

The scientists believe that their findings will help the understanding of conception, fertility, and gestational failures - research has already shown that couples with similar MHC genes had longer intervals between births, which could imply early, unperceived miscarriages.

Professor Bicalho says the intention is to follow up this work by looking at social and cultural influences as well as biological ones in mate choice, and relating these to the genetic diversity of the extended MHC region.

They expect to find that cultural aspects play an important role in mate choice, and do not subscribe to the theory that if a person bears a particular genetic variant it will determine his or her behaviour.

But they also suggest that the unconscious evolutionary aspect of partner choice should not be overlooked as their research shows that this has an important role to play in ensuring healthy reproduction, by helping to ensure that children are born with a strong immune system better able to cope with infection.

The research is being presented at the annual conference of the European Society of Human Genetics in Vienna today (Monday May 25).