Committed Relationship: You’re Wired For It (2011)

Pair bonding is a biological program not a cultural construct

Despite a colorful array of cultural differences, humans everywhere fall in love, attach emotionally for long periods, and feel betrayed when mates are unfaithful. These behaviors are innate, not the products of random cultural influences. To make this point another way: Most mammals don’t tattoo their mates’ names on their bums, and are not subject to fits of jealous rage.

Human Brains Are Built to Fall in Love, an earlier post, explained that pair bonding behaviors have neurobiological mechanisms behind them. Now, there’s more research evidence of our underlying pair-bonding programming. Predictably, it lines up with the evidence coming from the famous pair-bonding prairie vole. (More on that in a moment.) New York Times journalist John Tierney describes the new research this way:

The 21-year-old woman was carefully trained not to flirt with anyone who came into the laboratory over the course of several months. She kept eye contact and conversation to a minimum. She never used makeup or perfume, kept her hair in a simple ponytail, and always wore jeans and a plain T-shirt. …

Previous research had shown that a woman at the fertile stage of her menstrual cycle seems more attractive, and that same effect was observed here—but only when this woman was rated by a man who wasn’t already involved with someone else.

The other guys, the ones in romantic relationships, rated her as significantly less attractive when she was at the peak stage of fertility, presumably because at some level they sensed she then posed the greatest threat to their long-term relationships. To avoid being enticed to stray, they apparently told themselves she wasn’t all that hot anyway. …

Tierney adds that clearly,

Natural selection favored those who stayed together long enough to raise children: the men and women who could sustain a relationship by keeping their partners happy. They would have benefited from the virtue to remain faithful, or at least the willingness to appear faithful while cheating discreetly.

He also quotes U.C.L.A. psychologist Martie Haselton: “Women and men are affected by ovulation, but we [humans] don’t have any idea that it is what is driving these substantial changes in our behavior. [Such research] makes it clear that we’re much more like other mammals than we thought.”

How true. In voles, scientists are already uncovering the underlying neural mechanisms that regulate pair bonding behaviors, and sure enough, one of them is a mechanism that causes a male’s defensive aggressiveness toward unfamiliar willing females (once he has formed a pair bond with his main squeeze). This behavior obviously doesn’t happen for cultural reasons. It happens primarily because the neurochemical vasopressin increases in a key part of his brain. (By the way, it won’t necessarily keep Mr. Vole 100% faithful. Mrs. Vole, too, has been known to have a fling.)

Curious about the mechanics that dictate whether a mammal is capable of bonding? Turns out that in monogamous voles natural selection has reconfigured the distribution of oxytocin and vasopressin receptors in the brain’s limbic system. While all voles find sex rewarding, monogamous voles also get good feelings from a particular mate. The bonding mechanism, by the way, is the original addiction mechanism (which all other addictions hijack). This is why addiction can interfere with pair bonds.

Indeed, if scientists trigger the production of too much dopamine with artificial stimulation, an animal not only doesn’t bond, but also becomes aggressive toward all females. Could this help explain why some heavy porn users appear to be losing interest in real mates?

The vital point is that our pair bonding penchant arises from physiological events, not mere social conditioning. It evolved from the infant-caregiver mechanism, and the two mechanisms still overlap in the brain’s reward circuitry. So, even though many Westerners appear to be caught up in a chaotic hook-up culture for the moment, it doesn’t mean that we humans are, by nature, as promiscuous as bonobo chimps or that pair-bonding inclinations are superficial cultural constructs.

Keep in mind that human and bonobo evolution diverged some six million years ago. Our closest relatives are on our branch of the evolutionary tree, even if they’re no longer around. Somewhere along that branch humans became pair-bonders due to brain changes.

Although mammalian pair bonding is rare, the alterations that make a species pair bonders are not necessarily exotic. For example, pair-bonding prairie voles are so like their promiscuous meadow-vole cousins that scientists can convert a meadow vole into a pair bonder simply by prompting the expression of a single gene in his forebrain. (It increases receptors for vasopressin). In short, the behavior of our distant bonobo cousins is entertaining, but quite irrelevant when it comes to understanding the fundamentals of human mating.

“Ah, but look how promiscuous we are!” you’re thinking, right? Keep in mind two other points about our current hook-up behavior:

First, the studies we do in the West (generally using university students) are somewhat reckless in characterizing all of human behavior based on small slices of our rather unrepresentative culture. While strict monogamy is not the human norm, most mates still live in pairs. (Many cultures permit a man who can afford it to take another wife, but few can afford it.)

In short, if you are hooking up with multiple partners purely for recreation you could be an outlier. Your behavior is not typical human behavior—a point that is easily overlooked by Western researchers. For example, a 2007 study of 1,500 undergraduate men and women claimed to tell us “Why Humans Have Sex.” It found that many students had sex for recreation, not procreation. (Really??) In other cultures, humans are often quite adamant that sex is primarily linked to reproduction and building a family. Even sex with multiple partners may have stronger babies (“seminal nurture“) as its objective. (Lest readers jump to any misguided conclusions, I’m a ‘sex for recreation’ fan, but also a fan of the benefits of attachment.)

Second, the term “pair bonder” does not imply perfect sexual monogamy. It simply means mates are inclined to hang out together and raise offspring (known as social monogamy). No pair-bonding mammal species are entirely sexually exclusive; it would be an evolutionary handicap. So the reality that not all humans remain one hundred percent faithful for life and some of us have sex without attachment is not surprising. Variety also serves evolution.

Yet it’s useful to keep in mind that pair-bonder brains, including yours, are generally set up to attach to a mate. So, even if your milieu is wildly promiscuous for the moment, you have nothing to apologize for if you notice a hankering for a stable bond at the center of your sex life. The reasons lie in your brain, not your upbringing, and you can consciously tap this innate potential.

In our culture, which so prizes beauty and youth, it may seem downright odd that an aging couple could be more and more pleased by each other as the years pass. … If you know a handful of elderly couples, think about those among them who are still intensely drawn to one another. Watching them is evidence enough that attraction is not primarily based on attractiveness. … Seeing, touching, and hearing a devoted partner gains more and more power over time to trigger the release of [the bonding hormone, oxytocin].—Mark Chamberlain PhD

At least in a pair-bonding species like us.

See “The Ape That Thought It Was a Peacock:Does Evolutionary Psychology Exaggerate Human Sex Differences?”

(Excerpt) Pair Bonding

Pair bonding (or monogamy) is an extremely rare mating system among mammals, found in less than 5% of species (Kleiman, 1977). Nonetheless, it appears to be a central element in humans’ reproductive repertoire. It is therefore a curious fact that our dominant mating system is more like the typical mating system of birds than that of most mammals, including our nearest relatives, the Great Apes. In making this claim, it is important to be clear about three things. First, the claim is not that pair bonds necessarily last for life. In the absence of socially  enforced lifelong monogamy, most pair bonds last for months or years but ultimately dissolve (Fisher, 1992). Note, though, that a significant minority of pair bonds do last until the end of the lifespan, even in traditional forager societies that lack rigid strictures on divorce (see,  e.g., Marlowe, 2004).

Second, the claim is not that human pair bonds are always sexually exclusive. Most surveys suggest that considerably fewer than 50% of men or women in long-term committed relationships are ever unfaithful (Blow & Hartnett, 2005). Nonetheless, some are, and as a result, a certain fraction of offspring are sired by someone other than the social father (the best estimates place this at around 1–3%; Anderson, 2006; Wolf, Musch, Enczmann, & Fischer, 2012). Third, the claim is not that pair bonding is our one “true” or natural mating system. Humans exhibit all the mating systems found in other species, including monogamy, polygyny (one man, two or more women), and even polyandry (one women, two or more men; Murdock, 1967).

It is also not uncommon for people to engage in extra-pair mating, or to engage in casual sex before marriage or between long-term relationships. Different frequencies of each of these mating behaviors are found in different cultures and different historical periods. However, with the exception of long-term polyandry, all are relatively common, and thus all are plausibly part of the evolved repertoire of the human animal. Thus, our claim is not that pair bonding is humanity’s singular mating pattern. Our claim instead is simply that the pair bond is the most common setting for sex and reproduction in our species, that it has been for a long time, and that this has left a deep imprint on our evolved nature.