Monogamy may be held up as an ideal, but evolution has other ideas
Most of us know people in committed relationships, even lifelong marriages. And we also know stories about relationship transgressions, of partnerships tested or broken by infidelity.
As an evolutionary biologist who studies sex and relationships, I'm fascinated by these two truths. We humans make romantic commitments to each other - and some also break those commitments by cheating.
This might sound like a modern problem, but for me, it raises questions stretching far back in evolutionary time. Why did we evolve both a tendency to stay and a tendency to stray? If some among us will inevitably cheat, does that mean humans are hardwired for infidelity?
Answering those questions requires us to understand two key urges that propel our romantic and sexual behaviours as a species: the drive for a romantic bond and the drive for sexual novelty. Research shows that humans have evolved to seek secure partnership in the form of close pair-bond relationships, but it also shows, just as compellingly, that we've evolved a separate appetite for variety. Both drives are alive in us modern human beings, despite being at cross-purposes.
Yet some of us seek novelty more than others. In 2010, my research colleagues and I made a shocking discovery when we found that some people's genetic makeup does indeed create a predisposition to engage in infidelity. We conducted behavioural surveys and collected DNA from a sample of young adults. We then compared the subjects' reported behaviours against their genetic variations. Specifically, we looked at genes that influence the function of dopamine receptor cells in the brain.
In the brain, dopamine creates feelings of excitement and anticipation, and past research had shown that people who had the "long" version of specific dopamine genes, rather than the "short" version, tended toward thrill-seeking and risk-taking behaviour. This includes associations with alcohol abuse and gambling. In our study, we found that people who have the "long" version of the dopamine D4 receptor gene were 50% more likely to report having engaged in infidelity, and they were more likely to have done so multiple times.
Of course not everyone with the "infidelity gene" will betray their partners - biology isn't destiny, genetics being just one of a myriad of factors that influence us - but the study does suggest our ancestors evolved in such a way that some of the population would be more predisposed to sensation-seeking, and specifically sexual novelty, than others.
Why did we develop such a range of personalities and tendencies, and what does it mean for us today? Evolutionarily speaking, in any given population, it can be adaptive for some individuals to be inclined to take a risk and for others to play it safe, and for yet others to fall somewhere in between. This has shown to be true in all kinds of species: fish, for example. Studies show that in a given pond, it can be adaptive for the population to evolve some shy fish who stay safely in the deep end, and some bold ones who spend time in the shallows where, though they may get eaten, they also have access to more resources.
In other words, I think of infidelity as a phenomenon that can be understood at least in part as evolutionary adaptation - that is, a trait that can promote greater reproductive success. If some of our forebears were unfaithful, that might have led to more offspring, or more diverse offspring with different genetic mixes.
I still wouldn't say we're "hardwired" for infidelity, though. From an evolutionary standpoint, yes, we did inherit the potential to have sex outside our primary relationships. And yet in the modern social context, I think the more helpful framework is this: we're wired not for infidelity, but for sexual novelty.
Because let's not forget the other, equally powerful romantic drive our ancestors passed on to us: the adaptive motivation to form committed pair-bonds and protect them. This is a drive many species share: some animals attempt to enforce their mate's sexual fidelity by screaming, biting and clawing any interloper who gets too close to their pair-bonded partner; a behaviour known as "mate guarding".
When we look at human pair-bonds specifically, we see that we've evolved to seek out a trustworthy co-pilot and to protect that connection. I would argue that this was a way that humans managed uncertainty in our ancestral conditions, and still do today. In order to weather a storm with someone, you want to know that you can count on that person when the going gets tough.
Infidelity is by definition non-consensual; a betrayal of the expectations of the relationship. When the drive for novelty is expressed that way, it can erode and even destroy trust in one's partner and in the relationship. In other words, it threatens the pair-bond and can be devastating for both individuals.
I don't believe we've evolved to have to endure betrayal and pain. The better question, then, is this: what do we do with these two drives that, in the course of a long relationship, may grow to be at odds with each other? Can the human drive to seek novelty be explored in healthy ways?
I believe so. For some people, that may mean opening their relationship. For others it can mean finding ways to integrate novelty into their existing bond. This in itself can be a way to cultivate greater closeness.
Doing new things with your partner is great fuel for passion over time. In our research on long-term passion in couples of all sexual orientations, we found those who maintained sexual satisfaction engaged in a range of behaviours associated with intimacy and closeness. These might include giving or receiving a massage, going on a romantic getaway or planning a date night. And when it comes to a desire for sexual novelty, it could include talking about or acting out fantasies, trying new positions, or using a sex toy together.
On the surface, this might read as though the trick to sustaining interest is simply lots of sexual variety, but I believe the findings actually tell a deeper story about connection. Experimenting outside your comfort zone demonstrates interest in being intimate, and in so doing it communicates trust, vulnerability and attachment. That goes beyond any particular act of sex. Its power lies in the commitment to advancing passion, together.
Dr Justin Garcia is executive director of the Kinsey Institute and author of The Intimate Animal: The Science of Love, Fidelity and Connection (Penguin Life).
- The State of Affairs: Rethinking Infidelity by Esther Perel (Yellow Kite, £10.99)
- Secure Love: Create a Relationship That Lasts a Lifetime by Julie Mennano (Penguin, £12.99)
- Why We Love: The Definitive Guide to Our Most Fundamental Need by Anna Machin (W&N £10.99)