From Science:
Like meadow voles, some men just don’t seem to be built for monogamy, whereas others, like swans, mate for life. New research hints that some of the difference might be due to a single genetic variation. The gene in question, AVPR1a, governs a receptor that regulates the brain’s production of vasopressin, a hormone that contributes to attachment behavior with mates and offspring. A few years ago, scientists found that when they added extra copies of the AVPR1a gene to the brains of promiscuous meadow voles, the animals began acting more like monogamous prairie voles, spending more time with partners and grooming offspring. A similar role for the AVPR1a gene has been observed in chimps and bonobos.
Might such a simple switch be found in humans? A team led by Hasse Walum of the Karolinska Institute in Stockholm, Sweden, sequenced the AVPR1a gene in about 500 pairs of adult same-sex Swedish twins, all of them married or cohabiting for at least 5 years, and their partners. One variation of the gene was particularly common; about 40% of males had either one or two copies of a version–or allele–of the gene known as “334.” Although not simply an analog to the polymorphism found in prairie voles, allele 334 seems to have a similar effect on the stability of human relationships, as measured in interviews and questionnaires. The tests included a Partner Bonding Scale containing items that reflect affection and cooperation, such as “How often do you kiss your mate?” and “How often are you and your partner involved in common interests outside the family?” Scores on the test were significantly lower for the men carrying either one or two copies of allele 334 than for those without it.
More here.