WASHINGTON (Reuters) - The same gene that affects a rodent's ability to mate for life may affect human marriages, Swedish and U.S. researchers reported on Tuesday.

Men carrying a common variation of a gene involved in brain signaling were more likely to be in unhappy marriages than men with the other version, the team at the Karolinska Institute found.

Although they are not sure what the genetic changes do to a man's behavior, some other research suggests it has to do with the ability to communicate and empathize, the team reported in the Proceedings of the National Academy of Sciences.

"We never looked at infidelity in our study at all. What we have been focusing on is how strongly men bond to their partners," Karolinska's Hasse Walum, who led the study, said in a telephone interview.

Walum's team had been intrigued by previous research that showed one genetic difference seemed to explain why one species of vole formed strong pair bonds for life, while another mated promiscuously.

"Maybe this same gene will affect humans," Walum said.

They looked at a study of 552 pairs of twins and their spouses that detailed measures of parent and child relationships, marriage, personality and mental health of middle-class Swedes born between 1944 and 1971.

MONOGAMOUS RODENTS

The researchers tested the blood of men in the study, looking in particular for a gene that is similar in humans and voles. Called AVPR1A, it helps explain why prairie voles are monogamous and mountain and meadow voles are not.

The gene affects a brain chemical called neuropeptide arginine vasopressin and mostly affects blood pressure through the body's ability to retain water.

In humans, studies have shown certain variations of AVPR1A are linked with aggression, age at first sexual intercourse and altruism. One study suggested a link with autism, which affects the ability to interact socially, while another showed over-activation of the amygdala, the brain's emotional center.

Walum's team found that men with a gene variant, or allele, known as 334 earned low scores on their partner bonding scale, and were less likely to be married at all.

Men carrying two copies of 334 were twice as likely to have had a marital crisis in the past year. Their wives were much more likely to report dissatisfaction with their marriage.

"Fifteen percent of the men carrying no 334 allele reported marital crisis, whereas 34 percent of the men carrying two copies of this allele reported marital crisis," the researchers wrote.

More than 30 percent of the men who had at least one copy of 334 were unmarried, compared to 17 percent of the men who had no copies.

Walum said he has "no idea" how the genetic variant may actually affect a human being's behavior and stressed that larger studies must be done to test the association.

He would also like to test more unmarried men.