Like people, prairie vole males and females will have sex outside the pair-bond. Once male prairie voles are bonded to a female, and their reward centers reorganized, males will attack intruder females. We’ve also said that most females won’t bond to any other male if their partner dies or disappears. What, then, accounts for this paradox?
The male prairie vole’s vasopressin circuitry modulates his attachment to territory and space and helps forge his monogamous bond. A strange female (or a male, for that matter) who wanders in is violating the resident male’s homeland. He didn’t care before being bonded, but now he will attack. If he is out wandering — on a business trip — looking for food to bring home to the wife and kids, and happens to run across a female who’s been brought into estrus by another male, he may not be able to — or want to resist. His appetitive reward system switches on. Afterward, the vole won’t have a partner preference for her — that reorganization of his brain from liking to wanting still socially bonds him to his partner.
The female side of the affair we’ve just described is equally casual. Her estrogen is up because she’s been brought into estrus by the smell of her partner. But she’s alone, now, out looking for food. Or she’s sitting in the nest when her mate’s away. A strange male comes along. Driven by her appetitive desire circuitry, she allows him to mount. Then she comes home, or waits for her male to come home, with no change in her relationship.
Cheating voles and cheating people raise a question that’s central to the paradox of monogamy. If it’s sex you want, why not get it at home? Why take the risk?
Nature has played a trick on us. When a captive male marmoset is first introduced to a female, he becomes a very randy monkey. During the first ten days, he will have sex with his new girlfriend an average of over three times every half hour. The two form a monogamous bond. Sixty days into their relationship, they won’t be having sex at all. But they will be huddling together much more than they did at first. Essentially, says University of Nebraska’s Jeffrey French, who studied this pattern, within 80 days the marmosets “go from being young lovers to an old, married couple.”
Reflective of lowered motivation for pursuing sex, the males’ testosterone drops. So do their stress hormones. Their estrogen, meanwhile, rises. They’ve settled down.
Only 20 percent of married men age 40 to 49 said they have sex “two-to-three times per week.” Women showed similar trends. Many factors affect sexual frequency and motivation, but there’s little doubt that the falloff is neurochemically based. Married men have significantly lower testosterone than single men, just like the marmosets. They have higher estrogen and lower stress hormones. They are bonded and settled. They give back rubs that are just back rubs.
This lack of sex drive that accompanies long- term partnership is one half of a phenomenon discovered about 50 years ago and named, believe it or not, for Calvin Coolidge, the thirtieth president of the United States.
When physical passion disappears, there’s less glue to bind couples over the long haul of life, less excitement, less reward, often less closeness. If there are problems that had been buried under the passion, they can rise to the surface.
The second part of the Coolidge Effect, the rejuvenation of sexual appetite and performance, is a perfect example of the lure of novelty, and therefore an example of the lure of infidelity. It turns out that individual animals and people differ in just how powerful that lure can be when facing a conflict between desire and reason.
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Excerpted from “The Chemistry Between Us: Love, Science and the Science of Attraction” by Larry Young, Ph.D. and Brian Alexander. Copyright 2012 Larry Young, Ph.D. and Brian Alexander. www.thechemistrybetweenus.com.