when scientists injected oxytocin into the brains of female prairie voles, the animals formed a long-term bond with whatever male happened to be around. Similarly, when male voles that were genetically programmed to be promiscuous were given a gene that boosted vasopressin, they mated with one female exclusively, even though other receptive females were available. Vasopressin acted like a “good-husband hormone.” Dopamine does the opposite. Human beings who have genes that produce high levels of dopamine have the highest number of sexual partners and the lowest age of first sexual intercourse.
