The attachment system, fueled by the neurohormones oxytocin in females and vasopressin in males, drives animals, including humans, to pair-bond to rear their offspring as a team. Both hormones are triggered by orgasm, and both trigger dopamine release in reward regions of the brain. But all animals cheat, even when they form pair bonds. In most mammals, the bond lasts only as long as it takes to rear the young. Among prairie voles, science's favorite model of monogamy, knocking out the gene that codes for vasopressin receptors abolishes their penchant for pair-bonding. And implanting it in their notoriously promiscuous cousins, the mountain voles, leads the males to fixate on a specific female partner even when alluring others are abundantly available.
More recently, in a study of over 500 men, Swedish researchers found that variations in a gene that codes for vasopressin receptors in humans influences the very ability to form monogamous relationships. Men with two copies of a specific gene variant scored significantly lower on a questionnaire known as the Partner Bonding Scale and reported twice as many marital crises in the past year. Those with two copies of the variant were also twice as likely to be involved in outside relationships and far less likely to have ever been married than those not carrying the allele.