Men are more likely to be devoted and loyal husbands when they lack a particular variant of a gene that influences brain activity, researchers announced Monday -- the first time that science has shown a direct link between a man's genes and his aptitude for monogamy.
The finding is striking because it not only links the gene variant -- which is present in two of every five men -- with the risk of marital discord and divorce, but also appears to predict whether women involved with these men are likely to say their partners are emotionally close and available, or distant and disagreeable. The presence of the gene variant, or allele, also seems predictive of whether men get married or live with women without getting married.
"Men with two copies of the allele had twice the risk of experiencing marital dysfunction, with a threat of divorce during the last year, compared to men carrying one or no copies," said Hasse Walum, a behavioral geneticist at the Karolinska Institute in Stockholm who led the study. "Women married to men with one or two copies of the allele scored lower on average on how satisfied they were with the relationship compared to women married to men with no copies."
The scientists studied men because the hormone being examined is known to play a larger role in their brains than in women's brains.
The finding set off a debate about whether people should conduct genetic tests to find out whether potential mates are bad marriage prospects. Several independent scientists called the discovery remarkable and elegant but disagreed over whether such information ought to be used in making personal decisions about love and marriage.
Walum said that the presence of the allele increased the risk of conjugal discord, but that many other factors probably shape marital behavior. But he and other scientists said the study is the latest piece of evidence to show that biology -- down to the level of individual genes -- can play a powerful role in shaping complex human behavior.
In other words, if a man's culture, religion and family background each have a seat at the conference table that determines his attitudes toward marital fidelity and monogamy, his genes might well sit at the head of the table.
"There are many ways this information can help a man and his wife when they marry," said Helen Fisher, a biological anthropologist at Rutgers University who studies romantic love. "Knowing there are biological weak links can help you overcome them."
A man who knows he has this allele, she added, might be able to use the knowledge to ignore tugs of restlessness he might feel in his marriage: "You can say, `Oh, it is just my DNA, and I am going to ignore it.' "
The allele that Walum and a team of scientists studied in a sample of more than 1,000 heterosexual couples regulates the activity of a hormone in the brain known as vasopressin. It dictates how and where vasopressin receptors are situated in the brain. Effectively, said Larry Young, a psychiatrist who studies the genetics of social behavior at Emory University, brain receptors act like locks, and vasopressin acts like a key. The key works only when there is a lock; in the absence of a receptor, vasopressin cannot act.
About 40 percent of men have one or two copies of the allele. Walum, a PhD student, said that men with two copies of the allele had a greater risk of marital discord than men with one copy, and that men with one copy of the allele were at more risk of such discord than men with no copies. The study asked men in married or long-term relationships whether they had experienced relationship crises in the past year that were of such intensity that they considered divorce or splitting. The scientists also asked the wives and partners of the men what it was like to live with them, examining levels of affection, cohesion, consensus and satisfaction.
About 15 percent of the men without the allele reported serious marital discord in the past year, compared with 34 percent of men with two copies of the allele. Wives and partners of the men with two copies of the allele reported lower levels of satisfaction, affection, cohesion and consensus in the relationship than women married to men who had one or no copies of the allele.
Seventeen percent of the men without the allele were living with women without being married to them, compared with 32 percent of men with two alleles doing so.
The study was published in the Proceedings of the National Academy of Sciences.
Young praised the study, saying it extends a remarkable series of animal experiments he and other scientists conducted some years ago. They showed that the distribution of vasopressin receptors in the brain appears to predict why males and females form lifelong pair-bonds in one species but not in another.
The studies focused on two species that look nearly alike: prairie voles and montane voles. The first time a male prairie vole mates with a female, he forms a bond with her for life, breeding and raising successive litters. Male montane voles think of sex as a series of one-night stands; they are loners and do not bond with females or help raise offspring.
Young and others concluded that the difference between these species is because of the same gene variant that Walum studied. In the kind of experiments that cannot be replicated in humans, Young and others also showed that manipulating vasopressin receptors in vole brains can turn loner voles into devoted partners and fathers, and vice versa.
No one knows for sure whether the same mechanism is at work in humans. Although humans are evolutionarily distant from voles, there are many examples in nature that show that the action of genes is conserved across distantly related species. In the monogamous voles, Young said, bonding between animals seems to trigger vasopressin action in the brain's reward circuits. Not surprisingly, this prompts the animals to seek to bond with each other.
Geert de Vries, a neuroscientist at the University of Massachusetts in Amherst who studies vasopressin, hailed the new study and said it dovetails with work he and others have conducted that show vasopressin seems to play a much larger role in the brains of men rather than women. Beyond the immediate study on monogamy -- vasopressin seems to change how men, not women, behave in long-term relationships -- De Vries said there could be intriguing links between the Stockholm study and research into the causes of disorders such as autism.
"If you look at what is most prominent in kids with autism, the big difference is in social behavior," said De Vries, as he pointed out that autism is far more common in boys than in girls. "In this study, they are looking at social behavior related to marital status and the way men and women interact ... you could imagine variability in these alleles can contribute to autism."
All the scientists emphasized that more work needs to be done to replicate the finding, and to explore possible interactions between multiple genes and environmental factors. Partnerships in marriage and long-term relationships, moreover, involve many dimensions of behavior -- sexual desire, romantic love and the loyalty that Walum's study focused on.
"What this means is that some people will go into marriage with a stronger deck of cards," Fisher said. "But there are people genetically prone to alcoholism who give up booze and make a good marriage. No one is saying biology is destiny."
Fisher, who described herself as a romantic, said she would not reject a potential mate who has two copies of the risky allele. She paused, then added, "But I might not start a joint bank account with them for the first few years."