The Evolution of Romantic Love
Or, the sex lives of prairie voles and the rise(?) of polyamory
Prairie vole sweethearts
Prairie voles are adorable little rodents that live in the Midwestern US and along a strip of Southern Canada.
Unlike more than 90% of mammals, prairie voles are monogamous — they mate for life, and often beyond that. If a partner passes away, the remaining half of the couple rarely picks up a new partner. Monogamous prairie vole couples are also known for intense sex sessions (they frequently engage in 24-hour periods of plentiful sex, naps, and snack breaks1). Inspiring. Also, intimidating. *schedules extra cardio*
Curiously, a closely related species, the meadow vole, is not monogamous at all. They are more like enthusiastic participants in a free-for-all sex party. What’s going on here? Why is one vole species ferociously monogamous while a closely related one is profoundly promiscuous?
Hormones. Kind of.
Something peculiar has happened to the brains of our cutesy prairie voles: their ‘reward centers’ are stuffed with receptors for oxytocin and vasopressin. The first one is well-known for its link with maternal and social behavior, the second regulates several processes, including aggression and territoriality. So, if I were to simplify a complex story, we could say that prairie vole pair bonding is the result of evolution hijacking the females’ maternal behavior and the males’ aggression/territoriality to encourage long-term bonding.
There are parts of the story we don’t know yet. For example, if we genetically engineer promiscuous meadow voles to express oxytocin/vasopressin receptors in a prairie vole way, they become more monogamous, but the opposite is not true. If we remove the oxytocin receptors from the loyal prairie vole brain they don’t engage in wanton multi-partner sex parties. They still pair bond.
In an illustration of biology’s wonderful feedback loops, it’s not just about genes or hormones. Nature and nurture are two sides of the same coin, after all. If you isolate young vole pups from mom and dad, they don’t develop the same distribution of oxytocin receptors and, consequently, they are less likely to form long-term pair bonds as adults.
I have a confession to make. There’s a heartbreaking nuance in the prairie vole story I haven’t mentioned yet. They are socially monogamous, but not always sexually monogamous. Even our paragons of pair bonding have affairs. More specifically, remember those vasopressin receptors? The gene that codes for this receptor knows quite a bit of variation in male prairie voles and that variation correlates with fidelity. Prairie vole males fall into one of three categories: residents (the loyal guys), rovers (pair bonded but not shy of ‘extramarital’ affairs), and wanderers (unattached and screwing around with any interested female).
If you thought that sounded exactly like people, you’re right. With the risk of destroying a bunch of marriages… In human men, variation2 in the vasopressin receptor 1a gene correlates with pair bonding behaviors and marital problems.
Fortunately, people are more complex. Right?
The curious case of us
Humans are not prairie voles. But we are mammals.
To truly understand a complex behavior, we require an evolutionary and ecological context3. At the same time, we can’t ignore that human culture has a complexity that is unmatched in the animal kingdom. As a species, we’re just weird.
We can look at our primate cousins, but that’s tricky. We have (socially) monogamous gibbons, harem-having gorillas, matriarchal bonobos, and polygynandrous chimps (both males and females mate with multiple partners).
There’s another problem: human men have small balls.
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In contrast, the testicle of a male chimp is almost the size of his brain.
Why does this matter? Generally, in primates, testis size correlates with extrapair sex and intrasexual competition. If we look at primates and adjust for body size, human males have smaller testis than expected, which suggests a (relatively and on average) high level of fidelity within couples. Likewise, extrapair paternity (men having kids outside of their primary pair bond) in humans is comparatively low at around 2%, though the number varies somewhat depending on the study.
Looking at both evolutionary and cross-cultural evidence, this 2019 review on human mating concludes that,
… while there are many ethnographic examples of variation across human societies in terms of marriage patterns, extramarital affairs, the stability of relationships, and the ways in which fathers invest, the pair-bond is a ubiquitous feature of human mating relationships. This may be expressed through polygyny and/or polyandry but is most commonly observed in the form of serial monogamy.
For once, we’re more like prairie voles than like chimpanzees.
Wait. What does love have to do with this? Oh, you cynic. Fair point, though. Marriage is not a guarantee for love, love need not lead to marriage, and I’d never claim that romantic love can only exist in a monogamous bond. Yet, I don’t think we can separate the biology of pair bonding from the feeling of romantic love. Borrowing from this paper, which sets out to untangle the evolution of romantic love:
… romantic love is an evolved commitment device, [and] our review suggests that it is universal; suppresses mate-search mechanisms; has specific behavioral, hormonal, and neuropsychological signatures; and is linked to better health and survival.
A few neat examples in that paper show how some of the hormones that play a role in the pair bonding of other animals also affect humans. For example, people with higher oxytocin levels on their first dates are more likely to be together six months later. Ladies, exposing your man to an oxytocin nose spray might make him find you more attractive and - literally - keep his distance from other women. If we summarize a bunch of research, we find that the feeling of romantic love coincides with changes in serotonin levels and receptor density (‘sensitivity’ to serotonin), testosterone, cortisol, nerve growth factor, dopamine transporter density, and - in women - sex hormones known as luteinizing hormone and follicle-stimulating hormone. In some (but not all) hormonal and neural aspects, romantic love looks a bit like a (manageable) mania in bipolar disorder and the opposite of a depressive episode.
‘Love is manageable mania’ sounds so good I want to copyright it.
This excellent 2021 review has a beautiful (for a biologist, at least) conclusion about romantic love:
Romantic love is a motivational state typically associated with a desire for long-term mating with a particular individual. It occurs across the lifespan and is associated with distinctive cognitive, emotional, behavioral, social, genetic, neural, and endocrine activity in both sexes. Throughout much of the life course, it serves mate choice, courtship, sex, and pair-bonding functions. It is a suite of adaptations and by-products that arose sometime during the recent evolutionary history of humans.
Romantic love is not one thing; it affects many systems in our bodies and has evolved through various routes. It is hormones and brain activity and culture, all wrapped into a delicious box of sexy chocolates.
How many people do we want to share those chocolates with?
Holy poly
If my math is correct, a pair involves two people. Then again, one person can be part of different pairs.
Did someone say polyamory? I’m sure someone did; the internet has lit up with essays and discussions on the topic for the past few years. And I am intrigued. For now, intellectually. In practice, I’m one of those vanilla4 monogamy dudes.
It’s tricky to find good numbers on this, but roughly 5% of Americans are engaged in a form of non-monogamy, with around 16% of single Americans being ‘interested’. The numbers are higher in the UK — 7 and 23%, respectively. The general trend is upward, even compared to the early 2010s. Of course, we don’t know how much these numbers correlate with long-term commitments or if they reflect that people are simply more comfortable indicating ‘non-monogamy’ on a survey.
Let’s see how you beautiful people compare (no worries; completely anonymous).
Share for sample size. (Ninja marketing 101.)
Whatever the truth behind the numbers, it certainly feels as though people are generally more open to exploring romantic relationship forms that are not ‘traditional‘ monogamy, whether it’s hierarchical polyamory, relationship anarchy, solo poly, or kitchen table polyamory, to name a few. Despite what we might think, we can’t really pin this trend on one demographic slice of the population — it’s not tied to age, education level, income, religion, region, political affiliation, or ethnicity. Many of those surprised me, to be honest. Men seem to be slightly more interested, as are non-hetero people, though I am very suspicious of the male motivation here... Also, polyamory does not negatively affect psychological well-being and relationship satisfaction.
In general and with a mountain-sized grain of salt, people engaged in polyamory may have a personality style that includes higher openness and lower conscientiousness5. The research on polyamorous attachment styles is all over the place, but it seems that people in polyamorous relationships form specific attachments with each partner, so they’re pretty flexible in that regard. Keep in mind that all these studies are fairly small and mostly WEIRD (Western, educated, industrialized, rich, and democratic). Also, people often present themselves in an idealized way on surveys.
But is it natural?
Wrong question. As you can imagine, proponents of either lifestyle (monogamy vs non-monogamy) claim that their preference is more aligned with human nature. The problem with this is that ‘natural’ doesn’t matter here. The naturalistic fallacy is thinking that just because something is natural it’s good (or bad). It’s not. This is related to Hume’s famous is-ought problem. Just because a behavior is natural, doesn’t mean we ought to morally favor it.
Some books have tried to reconstruct primal human behavior to show that monogamy is unnatural. Sex at Dawn is a popular one, but it’s also widely discredited by researchers in the field of (human) behavior. Out of Eden by evolutionary biologist David Barash is a more moderate attempt. In my reading, Barash overplays - and this is weird to say as a biologist - the biology card. For example, he uses extra-pair paternity and sexual dimorphism as evidence, but we’ve already seen that extra-pair paternity in humans is low, and human sexual dimorphism is significantly less than in several of our primate cousins (and has probably been that way for a very long time).
Still, I find myself in agreement with his conclusion. Humans (biologically) land somewhere on the spectrum between strict monogamy and full polygamy, likely a tad closer to the first.
Which brings us back to our prairie voles. Not all of them are monogamous. Many of them are.
Can humans thrive in monogamous relationships? Yes. Can they thrive in polygamous/polyamorous relationships? Yes. Can they suffer in either? Sadly, also yes. Which one is more natural? Irrelevant.
Most humans can feel romantic love — what a profound privilege. How we express that, in which relationship configuration we nurture it, that’s up to each one of us. Go with your heart on this one.
Yes, there are strong cultural pressures and biological influences on who and how we love, yet the feeling, the dance of hormones and neurotransmitters, results in an ineffable subjective experience. The history of human artistic expression would have been poorer without it.
We would have been poorer without it.
Real love is promiscuously clicking buttons, right? Hugs.
In this Smithsonian Magazine long read on prairie voles, researcher Sue Carter, talking about behavioral observations of copulating prairie voles, says:
We had to put them on time-lapse video. No one could sit there for 40 hours!
For the sake of completeness, research looking at the actual time spent in copulation (what a job) finds that it is usually less than an hour in total. But, our rodents are not one-and-done kind of guys and gals; a dozen (or more!) brief rounds in a single day is not unusual. Prairie voles also “cuddle the shit out of their partners“. PDA, all day.
A polymorphic repeat sequence for all you geneticists. (This is technically relatively simple to test for… Genetic marriage counseling coming soon?)
I am biased; the ecology and evolution of individual behavior is my formal academic ‘expertise’.
A lot of people, however, forget that vanilla is a great option to sneak in chocolate chunks and bits of cookie dough.
Well, I’m one out of two, so the door has not slammed shut just yet…
Holy hell. This is incredible. Like -I’m going to reread this in the morning- incredible.
Good read, though the question whether someone is monogamous or polyamorous is (for most people) simply based on their conflict-constellation and no conscious choice.
Also modern biology is wrong in many ways and keeps ignoring the findings of Dr. Geerd Hamer.
Also there are many different ways to live polyamory. For most people it’s simply a way to run away from true commitment, while for some it’s a spiritual practice of learning to love more fully beyond romantic love (which is based only on hormones).