In modern society, couples are expected to remain sexually exclusive to one another. However, humans have pursued a variety of mating strategies throughout evolutionary and recorded history, suggesting that lifelong monogamy is not part of human nature. In the Post-Industrial Era, we are tending back toward the sexual patterns of our hunter-gatherer ancestors. This includes consensual non-monogamy, in which married couples give their partners permission to have extramarital sex.
In modern society, marriage is viewed as a union between two people who have vowed to be sexually exclusive to each other. This arrangement is also seen as the normal state of affairs for adults, and the vast majority of adults will enter into a monogamous relationship at some point in their lives, whether they officially register the marriage or choose to live in cohabitation. Despite the fact that nearly half of all marriages end in divorce, it is also generally believed that marriage should be a life-long commitment.
While lifelong monogamy is held as the norm, infidelity is fairly common, occurring in about a fifth of all marriages (1). It’s a major reason for divorce and for couples seeking counseling. Unfaithful partners are held totally at fault for breaking their vow of sexual exclusivity, and we are always shocked to learn that someone we know has been unfaithful to their spouse. Yet, the real mystery is not why there is so much infidelity in modern marriage, but rather why there is so little. After all, lifelong monogamy is simply not part of human nature.
Evolution of human mating
When we look at sexually reproducing species around the world, we see that lifelong monogamy is not a mating strategy that is commonly employed. Rather, the one-night stand is the norm, with mom left to raise the kiddies from that union. Pair-bonding for the purposes of raising offspring is the standard approach among songbirds and penguins, but the observed rate of extra-pair copulation is also quite high. Among the two species most closely related to humans – chimpanzees and bonobos – hippie-style free-love is the norm, with males and females mating for both reproductive and recreational purposes. It is probably the common ancestor of chimps, bonobos, and humans, living some six million years ago, that first came up with the idea of having sex for fun, since it is not a behavior we generally observe elsewhere in the animal kingdom (2).
Humans have an evolutionary history of about 2,000,000 years, starting with Homo erectus in Africa. By about 200,000 years ago, fully modern humans appeared in the fossil record. Throughout this time, humans and their near ancestors lived a hunter-gatherer lifestyle. They had fire, stone tools, and a sophisticated communication system, a sort of proto-language for Homo erectus that gradually evolved into the complex language of Homo sapiens (3).
Small bands of a hundred or so formed basic social units, and there was a strict division of labor along gender lines, with the males doing the hunting and the females the gathering. Over the vast time span of Homo erectus, human brain size expanded considerably, such that modern human babies are born extremely premature because their heads would not fit through their mother’s birth canal otherwise (4). Furthermore, human offspring take an extraordinarily long time to reach sexual maturity. This extended immaturity gives children the ability to learn about their culture, enabling humans to adapt more readily to the different natural environments they find themselves in as they spread across the planet.
At the same time, however, an extended childhood places demands on parenting that mothers alone would have difficulty meeting. Males who stick around and provide resources are more likely to have offspring that survive to adulthood (4). For their part, the females incentivized parenting behaviors in their mates by offering them frequent sex, even when they were pregnant or nursing their young.
Thus, pair-bonding evolved in humans for the same reason it had done so in songbirds and penguins, namely to greatly increase the likelihood that highly vulnerable offspring would make it to maturity. And yet, while pair-bonding for the purpose of raising a child is “encoded” in our DNA, lifelong monogamy most certainly is not. Since around the last third of the twentieth century, a marriage pattern known as serial monogamy has become more and more common. That is, we marry and have a kid or two, divorce, remarry and have another kid or two, perhaps even repeating the cycle three or four times. While conservatives bemoan the loss of traditional family values, evolutionary psychologists such as David Buss see modern serial monogamy as a return to the mating style of our hunter-gatherer ancestors, the reproductive style we evolved for (4).
A brief history of marriage
As we have seen, lifelong monogamy does not come naturally to human beings; in fact, it is a fairly recent invention. The institution of marriage arose along with the advent of agriculture around 10,000 years ago. Hunter-gatherers own no land, and they have little of value to pass on to their heirs. But farming changed all that. Not only did it tie us to the land, it also enabled us for the first time to accumulate durable wealth that could be handed down from one generation to the next. Thus, marriage was invented first and foremost as an economic arrangement for maintaining family property.
A woman always knows that the child she is giving birth to is her own, but for her husband there is the ever-lingering doubt that he has been cuckolded. Paternity uncertainty is also a problem for men in hunter-gatherer societies, as they may be providing resources for some other man’s offspring. However, the stakes are raised enormously in agrarian societies, where men want to be absolutely certain that their wealth goes to their biological offspring. Thus, women were expected to be exclusively monogamous, and they were sequestered and chaperoned to ensure they remained so. It was a different story for men, though, especially if they were wealthy enough to support any offspring that ensued from their sexual dalliances.
The abundance of resources that agriculture provided led to the institution of polygyny, or one man with multiple wives. Each wife, of course, was expected to be exclusively monogamous, but the husband not only had sex with each of his several wives, he could also take on concubines and seduce the servant girls. Among the peasantry, which made up the bulk of agricultural society, men also tended to be exclusively monogamous, but only because they could not afford a second wife. Moreover, the practice of polygyny meant that there were not enough available women for all men to marry. These wifeless men typically joined the military, where they met their sexual needs through prostitutes. In sum, varying patterns of long-term and short-term mating for both males and females prevailed during the age of agriculture, mainly depending on one’s gender and social class.
Monogamy as practiced in the West has its roots in Ancient Rome. Perhaps due to the democratic spirit of the Roman Republic, polygyny was banned and monogamy was the norm. Again, it was the woman who was expected to be exclusively monogamous, while monogamy for the man was more of a formality than an actuality. While his wife Calpurnia languished in Rome, Julius Caesar was off in Egypt making love to Cleopatra, even fathering a child with her. Although Christianity has its roots in Judaism, which practiced polygyny in Classical times, it settled on monogamy as it became the official religion of the Roman Empire. However, the other Abrahamic religion, Islam, still practices polygyny to this day.
The rise of modern marriage
The West, with its Roman-Christian heritage, has practiced monogamy now for more than two millennia. Women were only allowed to have sex with their husbands, while men of means could have extramarital affairs, with one caveat – do not touch women who “belonged” to someone else, namely the wives and daughters of other wealthy men.
Throughout this time, marriage was first and foremost an economic arrangement. Marriages were often arranged by parents with little regard for their children’s wishes. The purpose of marriage was twofold, first to consolidate wealth, and second to beget heirs to pass that wealth on to. If the couple happened to be fond of one another, all the more propitious. But even if the bride and groom met for the first time at the altar, there was still hope that a sort of love would grow between them over time, and in many cases it did. Of course, people still “fell in love” back then, as stories such as Shakespeare’s Romeo and Juliet attest (5). Nevertheless, infatuation was held as far too unstable a foundation on which to build a lasting economic union such as a marriage.
By the nineteenth century, however, we see the dilemma of marriage for love or money as a common theme in Western literature. And by the first half of the twentieth century, the idea that a prospective couple should be in love with one another had become generally accepted. Still, marital expectations were quite different from today. First, there were clearly established gender roles for the husband and wife, so there was none of the stress of negotiating responsibility for child rearing, income provision, and household tasks that plagues modern marriages. Second, attitudes toward marriage were different. In particular, spouses were viewed as life partners, with the marital coupling but one component within a much larger social network of relationships that included extended family, friends, and various associations within the larger community.
Young people today have a much higher standard when it comes to a potential spouse. They do not just want a partner to go through life with. Instead, they want a soulmate, the one and only person who will fulfill all of their emotional and relational needs. This is a pretty tall order. As marriage expert Eli Finkel explains, when couples can actually manage to become each other’s soulmates, the result is a marriage of rarefied bliss. And yet, for most of us, such high expectations are suffocating, and we blame ourselves when we fail to live up to an impossible standard (6).
We can point to a number of trends in modern society that have contributed to what Finkel calls the suffocation model of marriage. One easy scapegoat is Hollywood, with its endless supply of romantic movies on the common theme of looking for the one person in the world that is meant for you – your soulmate, in other words. However, Hollywood is a reflection of modern society, not a driver of it, so this explanation is too facile. We will have to drill deeper.
One social trend that has led to the ideal of the soulmate marriage is the gradual dissolution of the extended family during the Industrial Era. The extended family, with three or more generations living under one roof, was the norm throughout the Agricultural Era. However, by the end of the Second World War, the nuclear family composed of nothing more than husband, wife, and children had become the basic building block of Western society. The burdens of providing income, rearing children, and managing domestic tasks fell squarely on the shoulders of the young couple, rather than being distributed among grandparents, aunts, uncles, and grown siblings, as had been the case in the extended family. In short, the nuclear family structure sets up the mindset that husbands and wives have only each other to rely on.
A second relevant social trend is the gradual movement toward gender equality over the last century. The established gender roles of husband and wife have now all but disappeared. Many women are now the chief breadwinners of their nuclear family, and many men the primary caregivers of their children. Instead of entering into a marriage knowing what the expected roles are, each couple has to negotiate for themselves how to equitably distribute the burdens of marriage.
Among these is the burden of fidelity. In the past, only the women had to be exclusively monogamous, but in the gender-equal marriage, both husband and wife are expected to share the burden of remaining faithful. In other words, modern marriage achieves gender equality by restricting the sexual freedom of the man to conform to the limitations traditionally placed on the woman. Of course, gender equality in marriage could also be achieved by extending the man’s traditional sexual freedom to the woman, and that is the topic we turn to next.
In the soulmate marriage, our life partners are expected to meet all our emotional and relational needs. Some people find out, after marriage, that certain of these psychological needs are better met by friends or relatives, thus releasing some of the performance stress of modern marriage. Others learn this lesson as they go through couples counseling. So, if your spouse just doesn’t have your passion for bowling, or for deep philosophical conversations, there’s generally no problem with finding others who share your interests. But what happens when there’s a discrepancy in sexual desire? This is the challenge that all married couples will have to face, sooner or later.
In the infatuation stage of an intimate relationship, any difference in sexual desire is hardly noticeable, because both parties are getting enough sex to be generally satisfied. However, sooner or later, one spouse is going to want more sex than the other spouse desires, and it will eventually become a pivotal issue in the relationship. In the majority of cases, this is the man, but in a sizeable minority, it is the woman with the higher sex drive (7).
One solution is for the lower-sex-drive partner to acquiesce to the wishes of the higher-sex-drive spouse. In the traditional marriage, it was considered the wife’s duty to provide sex whenever the husband desired it. But in the modern marriage, sexual coercion is unacceptable. So what is the sexually frustrated partner to do? You can try to satisfy your sexual needs outside the marriage, but you vowed to forsake all others, and so if you go that route, it will have to be shrouded in secrecy. Here is the crux of infidelity in marriage: It is not the extramarital sex per se but rather the deception that is so damaging to the relationship. If only we did not cling so desperately to the soulmate model of marriage, one can reasonably argue, we could see that our relationship with our spouse could be so much more fulfilling if we would allow them to meet their needs elsewhere when we cannot meet them ourselves.
This observation leads us to a phenomenon known as consensual non-monogamy. CNM occurs when a couple acknowledges that they are unable to meet each other’s sexual needs and grant each other permission to seek out sexual gratification outside of the marriage. There are three main categories of CNM:
- Swinging, in which one couple swaps partners with another couple, often within the privacy of one of their homes, but sometimes in swingers’ clubs that can be found in all major cities.
- Open marriage, in which each spouse is granted permission to seek out additional sex partners, typically within certain restrictions as agreed upon by the couple.
- Polyamory, which involves a primary relationship plus additional long-term relationships that each spouse maintains; usually, all members of the polyamorous network are on friendly terms with each other.
Although the general consensus among the lay public and professional marriage counselors alike is that consensual non-monogamy can only lead to misery for all parties involved, the research actually shows that people in CNMs are just as happy in their relationships as those who are exclusively monogamous – but they are more sexually satisfied (8). Furthermore, people in CNMs display better communication skills, lower levels of jealousy, and higher levels of trust than those in traditional marriages. However, this does not mean that opening up the marriage is a panacea for sexual discrepancy issues. Rather, it means that people who are already good at communicating, low in jealousy, and high in trust are more likely to navigate the pitfalls of CNMs successfully.
Research suggests that about 20% of couples have experimented with consensual non-monogamy, although the rate of current practitioners is certainly much lower (8). This is because some couples who try CNM find that they lack the requisite characteristics to make it work. After all, most people have a hard time communicating about sexual issues, and their personal insecurities lead them to jealousy and trust issues with their partners. Yet, the irony is that the very qualities that enable couples to be successful in consensual non-monogamy are exactly the same characteristics that are required to make the soulmate marriage work – effective communication, low jealousy, and high trust. It is this triad of traits that enables a couple to effectively work through the problems that will inevitably arise in any marriage. Some couples will be able to negotiate a satisfactory solution to discrepancies in sexual desire within the boundaries of monogamy, while others will decide that opening the marriage – on agreed upon terms – is the best solution. In either case, what remains clear is that the key to a successful marriage is not in being the sole provider of all your spouse’s needs. Rather, it is in giving them the freedom to pursue self-actualization beyond the opportunities you can provide for them yourself.
- Murphy, A. P. et al., “A prospective investigation of the decision to open up a romantic relationship”, Social Psychological and Personality Science, 2020.
- Ryan, C. and Jetha, C., “Sex at dawn: How we mate, why we stray, and what it means for modern relationships”, 2012.
- Ludden, D., “The Psychology of Language: An Integrated Approach”, 2016.
- Buss, D., “The evolution of desire: strategies of human mating”, 2016.
- Shakespeare, W., “Romeo and Juliet”, c. 1596.
- Finkel, E. J. et al. “The suffocation of marriage: Climbing Mount Maslow without enough oxygen,” Psychological Inquiry, 2014.
- Kim, J. J. et al., “When tonight is not the night: Sexual rejection behaviors and satisfaction in romantic relationships”, Personality and Social Psychology Bulletin, 2020.
- Conley, T. D. et al. “Investigation of consensually nonmonogamous relationships: Theories, methods, and new directions,” Perspectives on Psychological Science, 2017.