Our modern era seems defined by our sexuality: who we are, who we love, and how we have relationships. We also seem to be in a constant state of changing norms and cultural expectations about relationships and family life.
In the United States, about 5% of the population is currently in a consensually non-monogamous relationship. Marriage rates are declining, with only 50% of U.S. adults having ever been married. This change, largely driven by young Millennials and Gen Z cohorts, is reflected in the high rates of singledom in the US: 31% of adults are single, and 41% of adults under 30 are single—and half of them aren’t looking to partner up any time soon. Finally, the U.S. tops the world in having the highest percentage of single-parent households at 23 percent.
What explains these modern cultural changes? Our evolutionary history offers us some insights.
The Evolution of Human Monogamy
We take for granted the fact that monogamy, loosely defined as two people pairing up in a sexually and socially exclusive way for a period of months to years, is not the norm in most mammals. It seems quite normal to us because most humans have a monogamous relationship at some point in their adult lives and such a relationship is culturally expected and rewarded. But only about 3-5% of mammals are monogamous.
Primates, however, are a bit more interesting: upwards of 20% of primate species are monogamous. This gives us a clue, from an evolutionary perspective, that perhaps there is something different about primates relative to other mammals that makes monogamy particularly beneficial. And there is.
Primates have some unique characteristics about their reproductive lives. In general, primates live long, have relatively long juvenile periods, and big brains that take time to mature. Humans take these traits to the extreme. We also have some unique traits such as women ceasing reproduction during menopause in mid-life and women not showing outward signs of ovulation like some primates do.
Humans also form monogamous pair-bonds, unlike primates who mate promiscuously (bonobos and chimpanzees), polygamously (gorillas), or solitarily (orangutans). But why? There are several documented benefits to humans mating monogamously across our evolutionary history.
First, men are relatively very strong and offer protection to women and their offspring. It’s an unfortunate fact of nature that males kill females’ offspring they did not father to avoid wasting their resources supporting non-genetically related progeny and to trigger the female back into a receptive reproductive state so they can sire their own offspring. Monogamy, it turns out, is an effective “anti-infanticide” strategy. It’s not the only one, but it is proposed as one effective way to reduce the risk of infanticide and is thought to be at least one strong driver of high pair-bonding rates observed across primates.
Second, a woman pairing up with a man gives him greater certainty that the offspring he is caring for are genetically his. After humans’ shift to pair-bonded mating patterns, paternal care become increasingly selected for, affording more resources investing into our highly dependent and demanding children. Although paternal care isn’t the norm in every human culture, it is far more common than in most mammals and primates, especially when there is less investment from other adult caregivers.
Third, women can up their reproductive output over the course of their reproductive lifespan with the support of a partner. There is evidence to suggest that partnered women can reduce breast feeding duration, which in turn reduces the spacing between births and results in more children overtime.
For men, the benefits of pair-bonding are a bit less clear considering that men don’t have to invest resources in gestation and breastfeeding like women do. But pair-bonding can be a beneficial reproductive strategy for men, too. By investing in one, relatively younger, female partner, he has reliable mating access, greater certainty of the relatedness of his progeny, and thus will invest more resources and have a greater probability of his offspring surviving—either from his invested resources or by protecting his offspring from infanticide.
Despite this compelling evidence for monogamy in humans, it’s less intuitive how this connects to the diversity of relationship arrangements we see across the world. Polygamy is still common in many regions, although most men can’t afford multiple partners and thus have no partners or only one. And in some regions, such as the Tibetan plateau, polyandry—a woman marrying multiple men (usually brothers)—is common. And as we’ve seen in the United States, more people are engaging in polyamorous relationships, or no relationship at all.
How do we get from monogamy to all this diversity?
Our environments have changed rapidly in the past 10,000 years since agriculture arose, changing some of the cues we use to determine how to proceed with relationships. Here, I’ll focus on U.S. cultural changes in the last 50 years. In this time, we’ve strongly decoupled sex and reproduction via effective birth control, seen the rise of women in high-ranking professional workplaces, and a profound shift in cultural attitudes toward casual sex, open relationships, and diverse sexuality. These changes have altered the inputs to our cognitive algorithm that drives our mating decisions.
Because women can control their fertility, no longer is sex with men a reproductively high-risk activity or long-term investment. Now that women can afford to live, work, and play financially independently of men, a relationship is no longer a requirement. With strong societal norms against, and punishment for, murder, risk of infanticide is essentially a non-concern for most parents. And with the broad cultural support for single parenthood, cohabitation, and diverse sexuality, people are free to relationship (yes, using it as a verb) however they please.
I’ve proposed previously that what determines whether we participate in monogamous relationships is a cognitive cost-benefit calculation of whether investing resources (time, energy, our genetic material) outweighs the costs of dissolving the partnership. In pre-civilization ancestral times, this calculation was likely more often on the side of staying in a monogamous relationship for at lest some long period of time, before maybe moving on to another monogamous relationship. But in today’s modern societies, women (and men) have far more flexibility in this cognitive calculation than ever before, resulting in many of the relationship (or lack thereof) patterns we observe today.
Not only does this calculation change broadly over time across societies, as described above, it can also change across the lifetime of an individual. As psychologist David Buss explains, we often hop around to different relationships over our lives. These cognitive calculations help determine when it’s time to move on and to whom.
These modern inputs to our cognitive mating algorithms, however, have implications for family structures and child rearing as well. In the modern west, the nuclear family—two parents raising several children within a household—is the idealized norm, but this norm is declining, as indicated by falling marriage rates and increasing single-parent households. But the nuclear family, historically speaking, is actually not the norm.
While it is true that humans are a pair-bonding species with fathers regularly investing resources and protection to their families, humans are also argued to be somewhat cooperative breeders. These “alloparents”—often female kin, like aunts and grandmothers who invest heavily in related children—have consistent and measurable impacts on child survival.
Again, however, these historical realities must be juxtaposed with our modern environments. Family members now move farther away from their extended families than what was typical in the past. It’s also far less common for families to reside in multi-generational homes where alloparents are daily factures in children’s development. In this context, two-parent households can be highly positive for children’s developmental outcomes.
What is most important for children is having stable developmental environments in which children form secure attachments to several caregivers, such as parents and grandparents, that best allow them to learn and develop. These types of environments will often involve two parents and reliable extended family members. But they also might look different than that. In fact, humans’ highly flexible behavior, even in mating, is what makes us who we are.
Nicole Barbaro is a Senior Communications Content Manager at WGU Labs, an Adjunct Professor of Psychology at Utah Valley University, and the Communications Officer for the Human Behavior and Evolution Society. You can learn more at www.nicolebarbaro.com and follow her on Twitter @NicoleBarbaro.
Editor’s Note: The opinions expressed in this article are those of the author and do not necessarily reflect the official policy or views of the Institute for Family Studies.