This Is Your Brain on Sex (27 page)

Dopamine and Fidelity

AVPR1A
is not the only gene that has been implicated in fidelity and relationship satisfaction. Recall that Justin Garcia found that the 7R+ variation of the
DRD4
gene, a gene related to risky behavior and addiction, was associated with a higher number of sexual partners outside a committed relationship. And it bears repeating that his study found no significant differences between men and women when it came to this effect.
⁸

Again, Garcia emphasizes that a particular genetic variation does not mean you will end up cheating on your partner; individuals who boast the 7R+ variation in the dopamine receptor gene may just be more motivated to seek out riskier sexual behavior, just as they might be more likely to jump out of airplanes, drive fast cars, or eat bizarre ethnic cuisine. “One of the big reasons people give for not cheating is that they don’t want to hurt the person they love. That can be enough of a reason for people not to do it,” said Garcia. “We’re cognitive creatures. We recognize there are consequences to our actions. No matter what our particular genetic makeup may be, we can use our frontal lobes and decide not to cheat.”

It is important to note that those evolutionary biologists were right on at least one count: our genes, particularly the ones that influence the vasopressin and dopamine systems, do have something to say about our attachments and perhaps our fidelity too. It is not, however, a deterministic effect. Just because you have a little kink in the genetic code involving one of these neurochemicals, that does not mean you are destined to cheat. It’s just not that simple. Furthermore there are a variety of other chemicals and neural pathways, some yet to be discovered, that may also play a role in whether or not we stray.

How Do We Define Monogamy?

To make it more complicated, our paragons of
monogamy, the little prairie voles, are hardly as pure as the driven snow. These animals may be socially monogamous, but some of both the males and females are still getting a little on the side. A study by Alexander Ophir, now at Oklahoma State University, found that although prairie voles remained with their pair-bonded partner, not all offspring actually were genetically linked to both parents.
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Ophir and his colleagues assessed twenty-six litters of prairie vole cubs in the field to determine paternity. Approximately 80 percent of the litters were sired by the male partner of the mother; the other 20 percent were not genetically related to the bonded male. You guessed it: the prairie vole version of the mailman sneaked in while hubby was away from the nest. And that guy was himself usually pair-bonded to another female. When it comes to vole love, there is a big difference between social and sexual monogamy, not to mention between the lab and an animal’s natural setting. It is the same in human beings; there is no biological evidence to suggest that every human, regardless of vasopressin receptor density in the nucleus accumbens or a particular variation in
DRD4,
is naturally monogamous. We may be culturally and socially encouraged to be faithful, but it is unclear how much sway that may have over our biological natures. That fun fact, unfortunately, brings us back to Roger’s argument, that cheating may just be an instinctual drive too fascinating to ignore.

Pharmacological manufacturers are putting great stock in vasopressin receptor antagonists; in fact, several are hard at work on a “monogamy” drug based on Young’s prairie vole research. Before you ask your doctor for a prescription (or invest in a few thousand shares of Big Pharma stock), remember that genes do not operate in a vacuum. In any monogamous relationship there are all manner of environmental variables related to happiness: how the kids are doing in school, how much money you have and how you spend it, how involved the in-laws are, and how frequently you are having sex. That just about covers what my ex-husband and I would bicker about in an average week. The environment also plays a big part in how our genes, including
the
AVPR1A
gene, are expressed.

“If you want to explain all the variation in human pair-bonding, you need to look a lot further than just a gene,” said Walum. “I think there is quite a bit of biology involved, but genes can only explain a bit about these behaviors. It is a variety of different factors working together—your genes, your culture, your age, your partner—that creates the true impact. You cannot say one of these things is more important than another.”

Monogamous Males Ask for Directions

Ophir, while working with his former advisor, Steve Phelps, also found that the ventral pallidum isn’t the only brain area linked to monogamy. The posterior cingulate, a brain region critical to processing spatial information, is also involved. Though prairie voles are monogamous, a small percentage of the species out in the field will never form a pair-bond. They simply wander about, opportunistically hooking up with females when they can. When Ophir and Phelps studied the brains of “residents,” monogamous males who do form a bond, and “wanderers,” those natural nomads and philanderers, they found no significant variations in the number of vasopressin receptors in the ventral pallidum. They did, however, find significant differences in the number in the posterior cingulate, as well as the number of oxytocin receptors in parts of the hippocampus, an area involved with memory.
¹⁰
These results led them to conclude that navigating space influences mating tactics—ergo monogamy. In order to successfully breed in the wild, prairie voles need to process not only social information about other animals but also spatial information concerning the location of those animals. Having the ability to recognize your mate does not do you much good if you don’t also have the ability to find her. It’s hard to argue with that logic. Males with lots of vasopressin receptors in the posterior cingulate are more likely to be residents. Low binding on this brain area makes for a wanderer. Ophir argues that these complementary circuits are critical for an animal to plan its best mating strategy, and they feed into the reward circuitry too. Also, interestingly, it seems
the number of receptors in the posterior cingulate is more likely to be passed down to offspring than those in the ventral pallidum, influencing whether future generations will become residents or wanderers themselves.
¹¹

“So is a wanderer always a wanderer?” I asked Ophir in front of the poster demonstrating these results at a professional conference. “Or will a wanderer one day find the right lovely female and settle down?”

He laughed. “We haven’t done that experiment yet. The natural history suggests the voles start off single, wander a bit, find that partner, and settle down. If the partner dies, most will remain single but will still mate with multiple females. Whether there’s some kind of reorganization of the brain at each of those steps, I just don’t know.”

Working Together

It is clear that no one brain area, no one chemical, is more important than the others. And with all this talk about vasopressin and dopamine, you might have forgotten that oxytocin also plays a key role in forming lasting pair-bonds. Shouldn’t it have something to say about fidelity as well? Some have hypothesized that though both sexes have both chemicals, oxytocin has more pull on females and vasopressin has a greater influence on males. To get a handle on female fidelity, all one had to do was identify the oxytocin equivalent of the
AVPR1A
gene. To date the studies have not borne out that hypothesis. When Sue Carter compared the effects of oxytocin and vasopressin on partner preference formation and social contact, she found both were necessary in both sexes. In fact, since the two chemicals can bond with the other’s receptor—meaning oxytocin can bond with the vasopressin receptor on the neuron and vice versa—it is possible that the two help each other out.
¹²

“I thought the data would come out that oxytocin was more relevant in females and vasopressin was more relevant in males. We all did,” said Carter. “Instead what we found was that both were important in both sexes. However, males produce more vasopressin in certain important regions of the brain associated with defensive behaviors. It is possible that this higher level of vasopressin helps to explain differences in the way that pair-bonds are expressed in males and females.”

The many ways oxytocin
and vasopressin may interact in the formation of a bond—and by extension monogamy—is still not known. Evolutionary changes in gene expression can be observed in different physical characteristics in races all over the world. Scientists are only beginning to understand how things we may experience in life, or even while still in the womb, can influence our later monogamous behaviors.

A Question of Epigenetics

Karen Bales, the neurobiologist from the University of California, Davis, studying prairie voles and titi monkeys, examines different developmental effects that may impact how an animal forms social relationships. “The idea is that an animal’s early environment, perhaps a stressor or maybe some kind of differential parental care, can possibly have an effect on the oxytocin and vasopressin systems in the brain,” she said. “Which, in turn, plays a role in how an animal forms a pair-bond later in life.” She hypothesized that exposure to oxytocin or vasopressin at an early age may provide such an epigenetic effect, in which an environmental variable changes the way genes are expressed in the brain. Women are often exposed to pitocin, a synthetic form of oxytocin, to help speed labor and delivery; therefore the discovery of an epigenetic effect taking place in voles would certainly have significant implications for people.

Bales and her colleagues injected a litter of prairie voles with a single dose of oxytocin on the day they were born. Given differences in development, this would equate more or less to the last month of gestation in a human child. The researchers then observed the voles, as well as a control group, while they aged. Once the voles reached sexual maturity, Bales’s group noticed an interesting sexually dimorphic effect, which was dependent on the amount of oxytocin to which the pup was exposed.
¹³
“When males got the oxytocin, that one little injection helped them to pair-bond faster and they showed a higher vasopressin receptor density in the ventral pallidum,” she said. But the same dose in females did not change their likelihood of pair-bonding. In fact, with a high enough dose of oxytocin, many of these females seemed to prefer male strangers to the father of their pups.

This implies that infidelity may not be a male biological imperative after all. These epigenetic
effects, the right combinations of genes and environmental influences, have the distinct potential to alter the way humans approach monogamy, regardless of gender. “The upshot here is that we’re seeing long-term changes in social behavior based on things pups were exposed to,” said Bales. “These early experiences are very powerful—just about everything you can do to a baby has the potential to change the brain.” So although pharmaceutical companies may be able to create a drug that can increase the number of vasopressin receptors popping up in our brain’s reward circuitry, it is very unlikely that this same drug could counteract all the other factors involved in fidelity—even if we were willing to give individuals this drug when they were still babies.

Bales’s laboratory is going beyond chemical exposures. These researchers are also examining handling behaviors, living arrangements, and even the effect of different types of parental responsibilities. In a recent study they found that prairie voles that helped raise their siblings had more vasopressin receptors in the amygdala, another part of the brain’s reward circuitry and an area of the brain implicated in emotional memory. So even if there is only a single gene at work—which is unlikely—all manner of different environmental variables may have an effect on how that gene is actually expressed in the developing brain.

Knocked Down but Not Out

Though I use my pal Roger to illustrate a stereotypical cheater, he cannot speak for all 22 percent of cheating men (and probably not for the 14 percent of creeping women either). Take a look at the people you know, the books you’ve read, the movies you’ve seen; no two cheaters are exactly alike. Love, lust, or simple opportunity may be at work in any given situation. Cheaters may share some qualities, the odd trait or two; they may even share some of the same reasons for straying outside their committed relationship. But their situations are not identical, and neither are their genomes or neurochemical makeup.

While neuroscientists have shown that many factors can influence the way
AVPR1A
may be expressed, what they have not yet looked at is how to factor in
individual differences. As I said, no two cheaters are alike, given their unique environments and genes. So no two individuals will express
AVPR1A
in quite the same way either. With so many variables at play, individual differences are crucial to understanding the resulting behavior.

“It’s pretty amazing,” said Larry Young. “You look at some of these animals and some have high levels of receptors, some have low levels, and then they show different behaviors. It may be a small effect and not a particularly good predictor of behavior. But it’s there.”

Young and one of his graduate students, Katie Barrett, are studying individual differences in the vasopressin receptor. Using a virus, Barrett is “knocking down” the gene to study its effects on animals. Unlike gene knockout techniques, whereby scientists totally silence a particular gene, knock-down methods allow the gene to be expressed, just not to its full potential. With precision the technique may allow neuroscientists to create groups of animals with different variations of gene expression, and then compare how they act with mates and their offspring.