On social media where everyone claims expertise in everything, a popular narrative persists: Men are inherently nonmonogamous, driven by evolution to “spread their seed,” while women are inherently monogamous, built for long-term love and attachment. Many evolutionary psychologists themselves and all the lay folks who misinterpret them, knowingly or unknowingly, often make these claims passionately–but is it true?

As with most questions concerning human nature and behavior, the answer is far more complex (and fascinating)!

The Evolutionary Imperative 

At the heart of evolutionary biology is the principle that all organisms, including humans of all sexes and genders, are shaped by evolution to pursue two main objectives: survive and reproduce. The ultimate goal is to maximize reproductive success—to leave behind as many offspring as possible who will themselves reproduce. (This latter piece is important – it’s not enough just to reproduce, you also have to parent those children so they survive to adulthood and continue the cycle.)

Of course, not every single individual is equally driven to reproduce. Many people opt not to have biological children for various reasons, a perfectly valid choice. Nevertheless, the vast majority do express a desire to reproduce. A recent nationally representative  study found that 89% of childless Americans aged 19 to 24 in 2015-2019 wanted to have children in the future, a slight decrease from 93.5% in 2002, but still a sizable number[1].

These individual differences notwithstanding, the drive to maximize our reproductive success is deeply ingrained in us, a foundational aspect of every species’ evolutionary legacy. If it weren’t, we’d all go extinct.

But while we’re all “programmed” with the drive to maximize reproductive success, we’re not “programmed” with any one specific strategy for how to go about this task. Although men do use the multiple partner strategy more often than women, and women use the single partner strategy much of the time, both sexes can and do often utilize both strategies (and a few additional ones).

Which exact strategy is most likely to result in maximum reproductive success for any one individual of a species depends on several sets of factors–biological sex is just one of them. Other factors include ecological conditions, sociocultural conditions, and an individual’s specific position within their ecological and socio-cultural environment.

Let’s look at each in turn. 

Biological Sex: Penises and Sperm vs Uteruses and Eggs

How reproduction takes place biologically in a species plays an important role in determining the kind of reproductive strategy that would be most beneficial for that species as a whole, and the different sexes of that species.

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All species that reproduce sexually (that is, by having two different organisms combine their genetic material to form a genetically unique offspring, as opposed to asexually, where one parent copies itself to form a genetically identical offspring) come in two distinct sexual forms – male and female – that differ from each other to some extent across several characteristics, including their reproductive anatomy and physiology*.

[*Some species have hermaphroditic members that have both male and female parts, or can change sexes from male to female or vice versa, but when reproducing, it’s always one set of male parts and one set of female parts combining genetic material.]

Among humans (and all other mammals) there are at least three major differences in reproductive anatomy and physiology between males and females that influence their reproductive strategies.

  1. Sperm vs Eggs. Males have a virtually unlimited number of sperm (up to 500 million in a good ejaculation!) that they produce from the day they reach puberty until well into old age. Females, on the other hand, have a fairly limited supply of eggs they are born with, cannot produce more over the course of their lives, and have a limited window of about 3 decades during which they can conceive a child.
    Fun fact: The sperm is the smallest cell in the human body; the egg is the largest.
  2. Pregnancy vs Nothing. Then, in order for any child conceived to make it into the world, males don’t have to do a single thing beyond the “wham, bam, thank you ma’am” act of depositing the sperm in the vagina, and are technically free to look for other available vaginas and uteruses as soon as they’re done depositing said sperm in the previous one (and they make it through their refractory period!). Females, on the other hand, have to invest 9 long months of pregnancy into carrying the embryo. And during this time they can’t make any more babies with anyone.
  3. Breastfeeding vs Nothing. Finally, for an infant to survive beyond infancy, females have to invest up to 2 years of breastfeeding (the average breastfeeding time during our long evolutionary history when we didn’t have busy schedules and baby formula), a time when they cannot conceive again because the hormones that help milk production suppress ovulation and menstrual periods (a form of nature’s own birth control known as lactational amenorrhea). In contrast, during those 2 years, males continue to be theoretically free to make many more babies with many other females.

To use the evolutionary psychology term, human males have a significantly lower “minimum parental investment” than females necessary to make babies, see those babies born, and keep them alive past infancy.

The result of this sex difference in minimum parental investment is that for males, the only theoretical limits on the number of children they can make is their access to willing fertile partners and the time needed to copulate. The more fertile females a male can mate with, the more children he can produce. (The world record holder is believed to be Sultan Moulay Ismail the Bloodthirsty who ruled Morocco from 1672 to 1727 and fathered somewhere between 880 and 1,170 children, thanks to 4 wives and 500 concubines. A 2014 study calculated this would’ve been possible if the Sultan copulated an average of about once a day.[2])

For females, on the other hand, the limit on the number of children they can make is primarily determined by whether they have access to resources to feed themselves and their offspring during the resource-intensive periods of pregnancy and breastfeeding. Multiple partners don’t necessarily help women make more children – she can technically have the exact same number of children with one fertile male as with 500 fertile males.

This is where the “men are designed to spread their seed” and women are designed to “nurture the garden” (or whatever silly analogy you want to use here) belief comes from.

But while this difference in basic numbers is indisputable and influential for men’s and women’s reproductive strategies, this is not the end of the story, especially for humans.

Why penises/sperm and uteruses/eggs are only part of the equation

What often gets lost in the translation of this “minimum parental investment” principle are three key things:

  1. Reproductive success is determined not by the number of children a person can make and give birth to, but by the number of children that survive until adulthood to themselves reproduce. These two are very different things. If a man spends too much of his time making babies and not enough time parenting those babies, there’s a good chance the kids he made won’t leave offspring of their own either because they die before they reach adulthood or because they are not of high enough “quality” to be desirable as partners. Either way, he loses the reproductive success game.
  2. Going for quantity of offspring requires significant investment of time, energy, and resources. One, to find willing fertile mates and mate with them until they get pregnant (remember, pregnancy can only occur on a few days each month). And two, to avoid getting killed or injured by any jealous partners or possessive fathers of those fertile mates. Given that most people’s resources are fairly limited, for many men who are not the Sultan (or his contemporary equivalents), trying for lots of babies with many women might instead result in no babies with any women.
  3. Finally, women also benefit from having multiple partners – not so much in the number of offspring, but in the quality of that offspring. Sorry guys, but not all sperm is made equal. All else being equal, having kids with multiple men, especially if at least some of them are of higher genetic quality, leads to higher overall reproductive success for a woman than having all her kids with one man of low or mediocre genetic quality.

All of this significantly changes the math of maximizing reproductive success, what we are all actually designed to do. It makes spreading the seed as far and wide as possible the winning strategy for some men under some circumstances, but not for all, or even most men, under many circumstances. And it makes going completely monogamous the winning strategy for some women under some circumstances, but not all or even most women under many circumstances.

Which reproductive strategy is going to be the winning one for any man or woman depends on several factors other than their biological sex.

The other parts of the equation

There are many other individual factors that change the reproductive math for people, but they can all be grouped into 3 main categories:

1. Ecological environment

The first set of factors have to do with the biological and physical environment in which any given man or woman finds themselves in their attempt to survive and reproduce.

How much food and other survival resources (warmth, shelter, tools) are available in your environment? How easy or difficult is it to access that food and other resources in general? How many predators, toxins, pathogens, and other things and people trying to kill you are present in your environment? The safer and more abundant an environment is, the less a man has to invest time and resources in his offspring’s survival, and the less dependent a woman is on a man for her and her offspring’s safety and well-being.

Another factor is simply how many potential mates are available in your mating pool. Are you in a small tribe that rarely comes in contact with other tribes, or do you live in a large and well connected tribe? To put it in today’s terms, are you in a small town or rural area that you rarely leave, or do you live in a major city while globetrotting around the world half the time?

2. Socio-cultural environment

The ecological conditions are relevant for all species. But for humans, there’s a whole other layer of social norms, rules, and expectations within a culture that play a massive role in what reproductive strategy makes most sense for any individual.

In cultures that promote monogamy and punish nonmonogamy for one or both sexes, monogamy is typically the only winning strategy for members of that sex. Anything else means serious risk of death or often equally deadly ex-communication and ostracism.

And of course, in virtually every culture across the world, women are punished much more harshly for stepping out of the monogamy box than men. In fact, for the past 12,000 years of human history, the vast majority of women have lived in societies where they didn’t have much or any independent access to resources. In other words, their survival and the survival of their children depended entirely on the men in their lives (first their fathers, then husbands). Given men’s interest in ensuring paternity of the children they are investing all those resources in (“paternity certainty”), women’s chastity before marriage and fidelity after marriage were tightly policed and transgressions against either norm severely punished. So, even if biologically many women could have benefitted from multiple partners, practically that was not a viable strategy for most until recently.

Now that it is becoming a viable strategy in many parts of the world, thanks to greater sexual freedoms, effective birth control to decouple sex from reproduction, and access to resources that doesn’t depend on men, many women take advantage of it. Cross-cultural research on over 200,000 participants from 53 countries, for example, shows that the more gender egalitarian a country is, the more interested in casual sex women are in that country. So women in, say, Denmark are still slightly less interested in casual sex than Danish men, but that gender difference is FAR smaller than the gender difference in say, Saudi Arabia [3].

3. Individual’s own position within the environment

Whatever ecological or sociocultural environment people find themselves in, there are huge individual differences in how appealing people are as potential mates in several ways. There are differences in:

a) genetic predispositions for physical appearance, health, and ability;
b) social standing on the dominance hierarchy (material and social status, power, and resources), and
c) personality characteristics (extraversion, kindness, charisma, humor, emotional stability, etc).

All of these factors make some people the kind of mating partner many in their environment want, while others are far less universally desirable.

If you’re attractive, powerful, and/or charismatic, going for quantity can be a winning strategy. The famous basketball player Wilt Chamberlain claimed to have had sex with 20,000 women, though was careful enough to not to make any babies.

If you’re on the other end of the attractiveness, resources, and/or extraversion spectrum though, your experiences will look far different than Wilt’s. Finding one partner to have babies with and dedicating resources to raise those babies might be your only realistic path to ensuring your genes remain in the gene pool.

Of course, just because you can, doesn’t mean you will. There are plenty of rich, powerful, attractive men who choose long-term monogamy, like Kurt Russell who’s been married monogamously to Goldie Hawn for 40 years, or Hugh Jackman and his wife of 24 years. Maximizing quantity is only one reproductive strategy that many men choose not to follow.

And it’s a strategy that many women have chosen to follow when given the opportunity. In this context, I’m always reminded of the story of this Chinese princess from the first century AD. As told by historian Stephanie Coontz in her fantastic book, Marriage, a History, apparently the princess argued that “she, like her brother the emperor, was entitled to a harem”. Although this was unheard of for women in China during that time, “her wishes prevailed, and she was assigned thirty male ‘concubines’” (p. 58).[4]

Beyond One-Size-Fits-All Solutions  

All in all, it’s true that males of all mammalian species, humans included, have the biological capacity to gain more from “spreading their seed” than females do. But many men across many societies have a lot to gain from a monogamous strategy, and many women, if given the chance, have a lot to gain from “spreading their eggs across different baskets” too.

And for many men and women, it’s a combined strategy that ends up being most evolutionarily beneficial: Find one partner to build a long-term relationship and family with, but sneak in an “extra-pair copulation” (AKA infidelity) if you can get away with it, to increase your chances of additional offspring or better quality/genetically diverse offspring. Or the age-old practice of serial monogamy, having one partner and children with that partner for some years, but then ending that relationship and repartnering.

Which of the several available reproductive strategies will be most advantageous for any one individual man or woman depends on a host of personal, societal, and ecological circumstances, biological sex being just one of many. Instead of adhering to oversimplified narratives about men’s and women’s evolutionary roles that social media “experts” state as gospel, it’s essential to acknowledge the nuanced and varied nature of human reproductive strategies.

And we didn’t even touch on another factor that plays a major role in all this – our individual differences in genetic predisposition toward monogamy vs nonmonogamy! But that’s a topic for another post…

References

[1]Guzzo, K. B., & Hayford, S. R. (2023). Educational experiences and American young adults’ childbearing goals: A research note. Journal of Marriage and Family

[2]Oberzaucher, E. & Grammer, K. (2014). The Case of Moulay Ismael – Fact or Fancy? PLoS ONE 9, 2: e85292.

[3]Lippa, R. A. (2009). Sex differences in sex drive, sociosexuality, and height across 53 nations: testing evolutionary and social structural theories. Archives of Sexual Behavior, 38, 5: 631-51.

[4]Coontz, S. (2005). Marriage, a History: From Obedience to Intimacy, or How Love Conquered Marriage. Penguin Group.