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A study of Swedish twin brothers found that differences in a gene modulating the hormone vasopressin were strongly tied to how well each man fared in marriage.
"Our main finding was an association between a variant of the vasopressin receptor 1a gene and how strong bonds men reported they had to their partners," said lead researcher Hasse Walum, of the department of medical epidemiology and biostatistics at the Karolinska Institute in Stockholm.
"Men carrying this variant scored on average lower on a scale measuring the strength of the bond compared to men not carrying this variant."
Women married to men carrying the "poorer bonding" form of the gene also reported "lower scores on levels of marital quality than women married to men not carrying this variant," Walum noted.
His team published its findings in the Proceedings of the National Academies of Science.
How the study was done
Walum's team first got interested in the role of vasopressin and bonding among males when studying a rodent, the vole. "Studies in voles have shown that the hormone vasopressin is released in the brain of males during mating," Walum explained.
Vasopressin activates the brain's reward system, and "you could say that mating-induced vasopressin release motivates male voles to interact with females they have mated with," Walum said. "This is not a sexual motivation, but rather a sort of prolonged social motivation."
In other words, the more vasopressin in the brain, the more male voles want to stick around and mingle with the female after copulation is through. This effect "is more pronounced in the monogamous voles," Walum noted. But voles and humans are very different species, so would the same effect hold true for men?
To find out, the Swedish team zeroed in the vasopressin 1a gene, which is shared by both species. Variations in this gene strongly influence vasopressin activity in the male vole, so Walum wondered if it might do the same for men.
To find out, his team looked for variants of the vasopressin 1a gene among 552 pairs of male twins enrolled in Sweden's ongoing Twin and Offspring Study.
What the study found
All of the men were currently in a relationship that had lasted at least five years, although about 18% of the men remained unmarried. The men were subjected to psychological tests assessing their ability to bond and commit, and the researchers also interviewed the men's spouses when possible.
They found that men with a certain variant, known as an allele, of the vasopressin 1a gene, called 334, tended to score especially low on a standard psychological test called the Partner Bonding Scale.
They were also less likely to be married than men carrying another form of the gene. And carrying two copies of the 334 allele doubled the odds that the men had undergone some sort of marital crisis (for example, the threat of divorce) over the past year.
All of these findings "make sense," said Dr John Lucas, a clinical associate professor of psychiatry at Weill Cornell Medical College in New York City. He said it's well known that genes help drive much of human behaviour, including mate bonding.
But the vasopressin 1a gene is likely not the only factor influencing a man's ability to form true and lasting bonds, he added.
"It's unlikely to be a single gene [at work] - it's likely to be multiple genes that are expressed incompletely and interact with the environment," said Lucas, who is also a psychiatrist at New York Presbyterian Hospital/Weill Cornell Medical Centre.
Too early for men to blame behaviour on genes
He pointed out that what psychologists call "temperament" - the individual palette of emotions and behaviours that even babies display - is probably "hard-wired" by our genetics. "But temperament, through training and experience, becomes personality," Lucas said. "And personality is a complicated situation, of course, and it involves the ability to commit."
So, it's too early for men to blame their inability to commit on a single gene, although Lucas guesses it's an excuse that's "certainly going to be used." For his part, Walum agreed that men and their spouses shouldn't read too much into the finding.
"Taken together, the effect of the gene variant that we have studied on human pair-bonding behaviour is rather small, and it can not, with any real accuracy, be used to predict how someone will behave in a future relationship," he said.
Walum also noted that the finding would probably not be applicable to women, since vasopressin appears to be tied to social bonding in males, but not females.
In a related study, also in the same issue of the journal, researchers at the Pacific Health Research Institute in Honolulu said they've identified a gene strongly linked to extended health and life span in humans.
The FOXO3A gene, involved in insulin signaling, is just the second gene ever found that is closely tied to longevity, the researchers said. In their study of Japanese-American men, those who lived to an average age of 98 had a specific variant of FOXO3A compared to men who died at younger ages, the team said. – (HealthDay News, September 2008)
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