Sex and relationships are tough, but luckily we’ve got behavioral psychologists, molecular geneticists, and evo-devo people working on that. In our world today, when communication is instant and the number and diversity of people one meets in a given moment would astound someone from a few decades back, relationships are nothing if not more complicated than they were, say….200,000 years ago. There are obviously huge social differences between us and our hunting-and-gathering ancestors but biologically, we’re pretty much the same. So–can we use our biology to help us understand our behavior…even when it comes to staying faithful to a partner?
There are currently some pretty heated debates about the nature of human relationships and whether or not humans, as animals, are biologically meant to be mated for life. Ryan and Jethá’s New York Times bestselling book Sex at Dawn: The Prehistoric Origins of Modern Sexuality discusses the evolutionary psychology of monogamy. They argue that many human physiological traits, such as copulatory vocalization (uh, sex noises), testicle size, and sexual dimorphism indicate that monogamy was not a common practice in our more primitive ancestors. Evolutionarily speaking, most animals are polygynous—for females, mating with multiple males usually means there’s a greater chance that offspring will be fathered by the most fit individual while for males, the more females he mates with, there’s a greater chance that he will father a greater number of offspring. Perhaps it is only modern bias, in which monogamy is the only really socially acceptable relationship choice, which has resulted in a misinterpretation of our ancestors’ sexual mores.
So why is it that human beings form life-long pair bonds? (Or at least…we try our best). It’s much more likely that sexual behavior shifted from a ‘promiscuous’ system in the hunter-gatherer stage toward long-term pair bonding with the development of agriculture, accumulation of wealth, and a different social structure. The authors of the book argue that the shift toward monogamy is fundamentally at odds with our animal nature. While carefully staying away from any moral pronouncement about what may be ‘right’ or ‘wrong’, the authors simply take the position that people should be more informed about the behavioral history of our species and go into relationships with a more complete picture of the choices they can make.
But what if a quick peek into our brains could tell us even more about the evolution of our species’ relationships? There are very few animal species who practice monogamy, mostly because usually, it’s more evolutionarily advantageous to mate with diverse members of the opposite sex. One of the lesser known monogamous species is, surprisingly, the prairie vole. Why surprising? Being rodents, voles reproduce fast and often, usually trying to gain an advantage by mixing up their mates. Prairie voles are one of the only species of rodent who mate with one specific individual, and scientists are just beginning to understand why—the answer lies in the structure of their brain.
When male prairie voles mate with same female a number of times in succession, their brain releases the hormone vasopressin, giving them a pleasurable feeling in the reward center of their brain. (They’re really cute too—they like to spend lots of time ‘cuddling’ with their chosen mate. Very scientific stuff, cuddling voles). This reward system makes the voles want to keep mating with the same female. These voles have probably developed a different kind of evolutionary advantage—when a male mates with just the one female, there is a higher chance that he will be the only father of all of her offspring. This behavior can also be explained physiologically: Prairie voles, unlike most other branches of their species, have a high number of vasopressin receptors in their brain, making this feedback loop powerful for them. Polygynous voles are lacking or have reduced version of the gene that codes for vasopressin receptors and so are unable to feel (or feel a greatly reduced version of) the positive sensation the results from monogamous mating.
But wait! It gets even better. Further experiments have shown that females can tell when males have increased levels of this receptor and will always pick them over males that have decreased vasopressin receptor levels. The hypothesis here is that males who mate monogamously are more likely to stick around to help take care of the kids (a much better choice of mate for the ladies). Larry Young and his lab at Emory University have even gone so far as injecting the gene that codes for vasopressin receptors into non-monogamous voles. The result? Voles who like to play the field suddenly became….monogamous. Adorable snuggle time with one specific mate ensued.
Could human brain chemistry give us more insight into the nature of our relationships? Could we genetically alter our mates to make them more faithful? Or is it not quite that simple? As always, social and cultural variables play a role in relationships and unfortunately, not everything is as black and white as science can make it seem. Humans are messy, and it takes a lot more than neurobiology to figure out why we behave the way we do. Let’s keep poking around in rodent brains and see what turns up.