Human Brains Are Built to Fall in Love

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What do your dainty eyeteeth mean for your love life?

Marriage dreamsHuman behavior varies a lot. As compared with other primates, we're heavily influenced by culture, religion, family upbringing, and so forth. As a consequence, it's logical to conclude that our fitful monogamy is purely culturally induced and not instinctual. (On the other hand, we readily seem to accept that promiscuous tendencies are wired into our brains.) 

In fact, we are programmed to pair bond—just as we're programmed to add notches to our belts.

By programmed, I mean that our brains are set up so that we engage in these behaviors with a lower threshold of enticement than we would otherwise. Both these programs serve our genes, as does the tension between them. For example, on average, we stay bonded long enough to fall in love with a kid, who then benefits from two caregivers. Then we may easily grow restless and seek out novel genes in the form of another partner. Italian research, for example, reveals that our racy "honeymoon neurochemistry" typically wears off within two years

Pair bonding is not simply a learned behavior. If there weren't neural correlates behind this behavior, there would not be so much falling in love and pairing up across so many cultures. The pair-bonding urge is built-in and waiting to be activated, much like the program that bonds infants with caregivers. In fact, these two programs arise in overlapping parts of the brain and employ the same neurochemicals. The Coolidge Effect (that sneaky tendency to habituate to a familiar sex partner and yearn for a novel one) is also a program. The fact that these programs often dominate one another doesn't alter the fact they both influence us.

Even when we override inclinations like these, they lurk. So it is that mates must often grit their teeth if they choose to remain faithful in the face of urges to pursue novel partners. And most humans are wired with powerful parent-child bonding impulses, even if they choose not to have children. It is a rare mother who does not bond with her kids (although it can happen if, for example, drug use has interfered with her neurochemistry). Similarly, people may choose never to engage in sex and orgasm, but groups of interconnected neurons are ready to give them a powerful experience if they do.

Again, such programs are present because of the physical structures in the brain—especially those that make up "the reward circuitry." This mechanism is activated by a neurochemical called dopamine (the "I gotta have it!" neurochemical). This is why falling in love, sex, nurturing a kid, and often pursuing a novel partner all register as rewarding.

Without this neurochemical reward, pair bonders wouldn't bother to pair bond. They'd settle into the usual, promiscuous mammalian program, in pursuit of its rewards. Predictably, there is evidence of unique brain activation in pair-bonding voles (compared with the non-pair-bonding variety). And there is data showing similar brain activity in pair-bonding primates. See: Neural correlates of pairbonding in a monogamous primate. Although more research is needed, it may be that pair-bonding mammals (unlike non-pair-bonding bonobos, for example) share similar neural correlates: neural networks, receptor type and specific neurotransmitters, etc. Neuroendocrinologist Sue Carter expressed this view: "The biochemistry [of bonding] is probably going to be similar in humans and in animals because it's quite a basic function."

While all mammals find sex rewarding, pair bonders also register the individual mate as rewarding. Thanks to this hidden pair-bonding program, our brains light up so we become infatuated. And our hearts ache when are parted from our sweetheart. Pair-bonding voles, too, show signs of pining when separated from a mate.

Need more evidence? Consider the hellish fury that arises when we are jilted for someone new. A cow, on the other hand, is quite indifferent if the bull that fertilized her yesterday does his duty with her neighbor today. Lacking the requisite neural correlates, she is not a pair bonder.

Proffered heartWhy should we care that we're pair bonders?

Given the fact that the urge to switch partners so often overrides our pair-bonding inclinations, shouldn't we continue to give this unreliable program scant attention? Maybe not. Even though our pair-bonding urge is clearly not a guarantee of living happily ever after with a lover, a better understanding of it may furnish important clues for relationship contentment, and even greater well-being. We don't have to conform to our genes' friction-prone agenda.

Let's consider some oft-ignored aspects of this program:

First, we may have come from a long line of pair bonders. A recent fossil find suggests that pair bonding could be the opposite of a superficial cultural phenomenon. The discovery of upright early human Ardipithecus (4.4 million years old) means that our line and the chimp line diverged long ago. Some researchers hypothesize that, because Ardipithecus males and females were about the same size, and the specimens do not have large, sharp canine teeth, it's possible that the fierce, often violent competition among males for females in heat that characterizes gorillas and chimpanzees was absent.

This could suggest that the males were beginning to enter into somewhat monogamous relationships with females—possibly devoting more time to carrying food (which would favor walking on two legs) and caring for their young than did earlier ancestors. See: Did Early Humans Start Walking for Sex?  In short, promiscuous bonobo chimps, our nearest living relatives, are really not so close. As non-pair bonders, they may have little to teach us about contented love lives.

Second, a happy pair bond offers humans sensations of deep, health-promoting satisfaction. When researchers measure happiness factors, a contented pair bond registers as one of the most important determinants of happiness. This may be a function of our pair-bonder wiring. Research shows that warm, comforting touch between mates appears to be protective of health and longevity. "Increasing warm touch among couples has a beneficial influence on multiple stress-sensitive systems." 

Affectionate contact between pair-bonding mates is apparently an exaptation of the same soothing contact that bonds infants and caregivers. Many popular articles emphasize that frequent orgasm benefits mates. Yet that assumption overlooks that we're wired to benefit from bonding and closeness themselves, quite apart from whether orgasm occurs. Clarity on this point can make staying in love more effortless than we thought.

In Are Pair Bonders More Vulnerable to Addiction? we'll look at a vulnerability that haunts pair bonders. 

[From Speaker Summary of talk by Larry Young, PhD entitled, "Neurobiology of Social Bonding and Monogamy"]

Prairie voles, like humans, are highly social and form long-lasting pair bonds between mates. This is in contrast to 95 percent of all mammalian species, which do not appear capable of forming long lasting social bonds between mates. Studies examining the brain and genetic mechanisms underlying pair bonding have revealed an important role for a few key chemicals in the brain in establishing social relationships. Oxytocin and vasopressin appear to focus the brain’s attention to the social signals in the environment. During pair bond formation, these chemicals interact with the brain’s reward system (e.g. dopamine) to establish an association between the social cues of the partner and the rewarding nature of mating. So why are some species capable of forming social bonds while others are not? Research comparing the brains of monogamous and non-monogamous species reveals that it is the location of the receptors that respond to oxytocin and vasopressin that determines whether an individual will be capable of bonding. For example, monogamous male prairie voles have high concentrations of vasopressin receptors in a ventral forebrain reward center that is also involved in addiction. Non-monogamous meadow voles lack receptors there. However, if receptors are inserted into this reward center in the non-monogamous meadow vole, these males suddenly develop the capacity to form bonds. These studies also suggest that pair bonding shares many of the same brain mechanisms as addiction. Genetic studies have revealed that DNA sequence variation in the gene encoding the vasopressin receptor affect the level of receptor expression in certain brain regions and predict the probability that the male will form a social bond with a female.

Recent studies in humans have revealed remarkable similarities in the roles of oxytocin and vasopressin in regulating social cognition and behavior in vole and man. Variation in the DNA sequence of the human vasopressin receptor gene has been associated with variation in measures of romantic relationship quality. In humans, intranasal delivery of oxytocin enhances trust, increases gaze to the eyes, increases empathy and enhances socially-reinforced learning. Indeed it appears that stimulating the oxytocin system in humans increases the attention to social cues in the environment....