Question: What makes for a close couple relationship? Common interests and tastes? Identical cultural roots? Harmonious pheromones? All may play a role, but science suggests we also look at variants in the particular gene that encodes the receptor for an important brain-signaling hormone, arginine vasopressin. One of the human cell receptors that detect arginine vasopressin takes slightly different forms, with the variants controlled by genes we inherit from our parents. A recent study shows one of these variants (or rather, the lack thereof) to be associated with a distinctive kind of pair-bonding behavior in men—though, interestingly, not in women.
Findings: Men who have one copy of the gene variant 334, compared with similar men without 334, were substantially less likely to have attributes linked to close couples (high frequency of kissing one's mate, for instance, or pleasure in being close to another person). They were also more likely to have marital problems and to be unmarried. In men carrying two identical copies of 334—that is, homozygous men—all three of these effects were even stronger.
Method: The study's subjects were more than 500 Swedish same-sex twin pairs and their spouses or partners. (All the participants were members of couples that had been together for at least five years.) The researchers gauged preference for physical closeness by asking whether the subjects were put off when other people come too close. They asked how often the couple did things in common outside the family. They asked whether either partner had spoken with a close friend about divorce or separation and about marital crisis.
Link: Similar effects on the bonding of pairs, involving a similar gene, have been extensively studied in voles (rather cuter rodents than most). Prairie voles are famously treasured by social conservatives because they monogamously pair off for life, share in the care of offspring, and usually remain a widow or widower if one partner dies. (It turns out the social monogamy of the prairie vole doesn't always translate into sexual monogamy, but you can't have everything.) In contrast to the prairie vole, the montane vole doesn't go for such cozy co-habiting. Instead, the males act like frat boys. For voles, genes also control the pair-bonding behavior of males, and the gene in question is also the DNA sequence that codes for an arginine vasopressin receptor.
Conclusion: Genetic studies like this one—and others involving genetic variants that seem related to risk-taking behavior and sexual desire—zero in on a behavior that's too complex and interactive to predict in individuals, but they also have important effects on how we live our lives. Intuitively, we're often aware of these kinds of genetic links. (Think how often you've seen a child do something and said to yourself, "Of course, that's just like his Dad!") At the same time, it's terribly difficult to separate the genetic influences from imitation and nurture. For years I wanted to reject genetic influences on behavior because I hated the determinism I thought it implied. I'm easier now since I've come to realize that the subtlety and complexity of the interaction between nature and nurture still means that how any one person acts is as unpredictable as the path of a leaf tumbling in a wind.