Research presented last month by a team from Brazil shows that married couples have less MHC (Major Histocompatibility Complex) similarity than randomly paired individuals selected from the same database. The study, led by Professor Maria da Graca Bicalho at the University of Parana, Brazil, compared MHC genes in 90 husband-wife couples with MHC genes in random couples generated from the database.

MHC has previously been shown to play an important role in immunity and fertility and female preference for MHC-dissimilar mates has been shown in several species including humans. It is thought that parents with dissimilar MHCs could produce offspring who have superior immune systems and thus fight off infection more easily.

'It may be tempting to think that humans choose their partners because of their similarities,' said Professor Bicalho, '[But] 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.'

Cultural and social factors are also known to play a role in mate selection and these were not examined in this study. Researchers are not yet at the stage of being able to predict the success of partnerships based on genetic similarity. Indeed, the ethics of doing so is far from clear.