LONDON, (Reuters) – When it comes to choosing a mate,  opposites really do attract, according to a Brazilian study that  found people are subconsciously more likely to choose a partner  whose genetic make-up is different to their own.

They found evidence that married couples are more likely to  have genetic differences in a DNA region governing the immune  system than were randomly matched pairs.

This was likely to be an evolutionary strategy to ensure  healthy reproduction because genetic variability is an advantage  for offspring, Maria da Graca Bicalho and her colleagues at the  University of Parana in Brazil reported.

“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,” Bicalho said in a  statement.

Scientists said it was not clear what signals attract the  body to people who are genetically dissimilar to themselves, but  suggested body odour or even face structure could play a role.

Many researchers have found evidence than animals are  attracted to members of the opposite sex with differences in  major histocompatibility complex or MHC, an immune system factor  that also plays a role in having healthy offspring.

Bicalho, who will present her findings at a conference of  the European Society of Human Genetics in Vienna today, said  the team compared genetic data from 90 married couples with data  from 152 randomly generated control couples. They found the real couples had significantly more  dissimilarities in MHC.

“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,” they wrote  in a summary prepared for the meeting.

“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,” Bicalho said.

“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,” she added.

Previous studies have suggested animals may use body odour  as a guide to identify possible mates as being genetically  similar or dissimilar, she added, but other physical factors may  also be involved.