The bond of true love may be forged in the genes as well as in the mind, researchers have found.
A comparative survey of couples suggests people are more attracted to those who have very different immunity genes from their own, even though they are not aware of it.
The genes in question play a major role in the immune system's ability to fight infections, but they are also thought to leave a lingering trace in the scent of people's body odour.
The scientist who led the study believes humans have evolved to sniff out partners who have different immunity genes because they tend to produce healthier children with stronger immune systems.
"It may be tempting to think that humans choose their partners because of their similarities," said Maria da Graça Bicalho, a professor of immunology at the University of Paraná in Brazil. "[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."
Bicalho's team looked at a group of genes known as the major histocompatibility complex (MHC), and noted down how much the genes varied between 90 married couples and 152 fictitious couples paired up at random by a computer.
"If MHC genes did not influence mate selection, we would have expected to see similar results for both sets of couples," said Bicalho, "But we found that the real partners had significantly more MHC dissimilarities than we could have expected to find simply by chance."The MHC region is made up of a large number of genes on chromosome six, and has been found in most vertebrates. As well as being linked to immunity, the group of genes are also thought to play a role in fertility.
The flip side of the study suggests humans have evolved to find people with similar immunity genes unappealing. This would have the effect of reducing inbreeding, which can have serious medical consequences for a couples' children.
Earlier studies have shown that couples with similar MHC genes have children further apart, which might be due to the woman having more early-stage miscarriages that go undetected.
"We expect to find that cultural aspects play an important role in mate choice, and certainly do not subscribe to the theory that if a person bears a particular genetic variant it will determine his or her behaviour," said Bicalho. "But we also think that the unconscious evolutionary aspect of partner choice should not be overlooked. Our 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."