Human Reproduction as Prisoner's Dilemma

The decline of marriage in the West.

Jan 21, 2025

Written by Arctotherium.

The core problem marriage exists to solve is that it takes almost twenty years and an enormous amount of work and resources to raise children.

In hunter-gatherer societies, it takes almost twenty years for the average person to become a net producer of food. Until then, they are dependent on others, mostly their parents, who have a direct genetic stake in their survival. (The numbers are similar for agrarian societies.) Source.

This makes human reproduction analogous to a prisoner’s dilemma. Both father and mother can choose to fully commit or pursue other options. In this context, marriage provides a framework for encouraging, legitimizing, and stabilizing commitment.1

Defection

If human reproduction is analogous to a prisoner’s dilemma, what does defection look like?

A natural consequence of sexual reproduction is adversarial reproductive strategies. Females must expend significant biological resources but can be certain of maternity, which allows maternal investment to pay off as soon as a child is born. Males don’t need to spend much biologically, but can’t be certain of paternity2, making paternal investment inherently risky. By default, this leads to two idealized strategies that maximize the benefits for each sex. In humans, the idealized male strategy is to have as many wives/exclusive sex partners as he can afford3 – plus opportunistic extra-pair couplings, consensual or otherwise. The idealized female strategy is to secure investment from a man, while retaining the option to trade up for a more attractive man4 at any time – plus opportunistic hidden extra-pair couplings with attractive men.

Both of these strategies greatly damage the interests of the opposite sex. Cuckoldry is equivalent to death for a man from a Darwinian perspective. And in evolutionary environments requiring paternal investment, abandonment is equivalent to death for a woman. Losing decades of youth and all of the resources invested in a pairing is devastating for both.

Men defect/women cooperate

A defect/cooperate society, in which men act to secure the collective interests of their sex without regard for those of women, looks like Meiji Japan (which was monogamous) or early 20th century Arabia (which was polygamous). In these societies, women are effectively property. Divorce is common (Meiji Japan had the highest divorce rate of any country with records in the world) and devastating to women, who lose their children, economic status, virginity and youth.

Late 19th century Japan had divorce rates several times higher than those of northwestern Europe or the Anglosphere. Source.

However, a simple comparison of divorce rates between different marriage regimes is misleading. Since divorce is at the whim of the husband in defect/cooperate societies, men are free to invest in their family and children – which is not true in societies where men can lose their families against their will. Furthermore, paternal certainty in defect/cooperate societies is guaranteed, which means such societies can be highly functional, especially when they have monogamy5 to reduce male intrasexual competition. Meiji Japan was phenomenally successful economically, demographically, and territorially until destroyed by an overwhelming outside force.

Men defect/women defect

A defect/defect society looks like the most primitive parts of sub-Saharan Africa or the Amazon rainforest. Women sleep around, while adult men prey on women and children and regularly kill each other for access to women. As men can have multiple wives and wives are not loyal, there is no respite from intrasexual competition; you can always be replaced.

Among the Himba of Southern Africa, the vast majority of married men and women have extra-pair sex partners, and 48% of children are not the husband’s. Source.

Without paternal certainty, men have no investment in the future and spend their time fighting, dancing or resting rather than working. Economically, these societies are desperately poor and largely incapable of collective action. In war, they shatter like glass when faced with an enemy that expect chastity and fidelity from women.

Defect/defect societies tend to be ones without a history of plow use pre-1500. Plow agriculture requires male labor to sustain a family, which meant that groups that didn’t enforce paternity certainty could not survive. Defect/defect societies are reliably the poorest, most technologically backwards, and most violent. Source.

Men cooperate/women cooperate

The Western solution6 to the prisoner’s dilemma can be summarized as follows:

Monogamy.
Marriage by mutual consent.
High standards of premarital chastity (especially for women) and fidelity.
Divorce is difficult: the marriage contract can be created by mutual consent, but cannot be unilaterally dissolved.
Men materially support their wives and children.

Let’s deal with each pillar in turn.

Monogamy. Polygamy is a natural attractor state for humans, since it satisfies the desires of powerful men to have multiple wives and the desires of women to have elite husbands. Monogamy requires both elite men and many women to sacrifice their desires. In exchange, it provides strong checks on negative-sum intrasexual competition. Powerful married men are not constantly on the lookout for another wife and can devote their efforts to other pursuits, while less powerful men, who would be shut out of marriage in a polygamous society, have fewer evolutionary incentives to stab their compatriots in the back. The result is a much more cohesive and powerful society. But it’s not just male coordination that benefits from monogamy. As Joseph Henrich (2020) notes:

Because of how monogamous marriage influences social dynamics and cultural evolution, inhibiting female choice—by prohibiting women from freely choosing to marry men who are already married—results in both women and children doing better in the long run (on average). This occurs because of how the social dynamics unleashed by polygyny influence household formation, men’s psychology, and husbands’ willingness to invest in their wives and children.

Rather than invest in additional wives, men in monogamous societies invest in their original wife and children, with the result that almost everyone is better off.

Marriage by mutual consent. This comes from Catholic doctrine (“What therefore God hath joined together, let not man put asunder”) and particularly benefits women, who cannot be forcibly married against their will.

High standards of premarital chastity (especially for women) and fidelity. This ensures two things. First and most importantly, it ensures paternal certainty and minimises sexual jealousy. Second, it makes marriage the gateway to sex, which strongly encourages young people to get themselves into the long-term pair bond that is the ideal environment to have and raise children.

Divorce is difficult: the marriage contract can be created by mutual consent, but cannot be unilaterally dissolved. This allows for greater specialization of labor within the household and greater investment in “relationship capital,” including children. It also shifts the balance of power within relationships to the more committed partner.

Men materially support their wives and children. They are not expected to provide for children that are not theirs, nor for children born outside of wedlock. If men do not have access to their children, they are not expected to support them unless they are personally at fault. This should be seen as a contract between the prospective husband and wife.7 Like any reasonable contract, both parties are giving something up and getting something in return.

Breaking the bargain

Pillars one and two of this solution remain intact. Western countries are still formally monogamous and marriage is still by mutual consent. (Though without any broad religious or philosophical justification, this may change. Popular support for legalizing polygamy has surged in the 21st century US.) But pillars three to five were systematically demolished in the mid-20th century, mostly between 1960 and 1980.

Chronology of major events in the “women’s rights” movement in the US. I include the scare quotes because this movement should be seen as a zero-sum struggle for political and economic power (affirmative action for women is correctly included as a major achievement by Goldin). Source.

This is a chronology of laws because laws are highly legible. But what matters is people’s behavior. Sufficiently strong social norms are indistinguishable from laws, and laws are interpreted by judges, meaning legal practice may change dramatically without any significant change to official statutes.

The sexual revolution

There no longer exists any expectation of chastity until marriage. But within marriage, there’s still the expectation of fidelity, though without any enforcement mechanism beyond social stigma. This reverses the sexual incentive to marry. Rather than a gateway to sexual opportunity, marriage has become a chain restricting it.

Note that these numbers are lower than in reality because they are self-reported. When hooked up to a lie detector, women claim about one more sex partner than they do when asked normally (Alexander and Fisher, 2003). Source.

Whereas the pre-70s dispensation aligned sex drives with the pair bonding desired for raising children, the present one sets them in conflict with each other.

Unilateral Divorce

The most obvious effect of unilateral (i.e., no fault) divorce is more divorces.

us_divorce_and_marriage2 — There’s much talk about the decline in US divorce rates since the 80s, but the reason for this is that marriage rates have also dropped. Note that the rise in divorce post-WWII corresponds to marriage becoming much *less* selective, while the spike post-no fault divorce corresponds to the institution becoming much *more* selective. Source.

It’s worth considering what this actually means. Each divorce represents years of effort and investment wasted, and often perpetual and unwanted obligations. It often means broken families and the misery that accompanies them. In the United States, only 52% of first marriages survive the 20 years required to raise children.

This table also shows why marriage has become an upper-class institution (per Charles Murray). Society no longer works to keep marriages intact, meaning it requires a certain amount of foresight and lack of impulsiveness to maintain one. Source.

But unilateral divorce doesn’t just destabilize marriage. It also changes the power dynamics within marriage from favoring the more committed partner to favouring the less committed partner. Hence, “under unilateral divorce, the distribution of resources within marriage favors the spouse who wished to divorce” (Reynoso 2024). In addition to destabilizing marriage, unilateral divorce incentivizes poor behavior within it, since the threat of ending the marriage on unfavorable terms for the undutiful partner no longer exists. This “weakens the bargaining power of dutiful partners who wish their marriage to continue or who wish to end their marriage because of serious mistreatment by the other partner” (Rowthorn 1999).

Unilateral divorce is sometimes portrayed as an advance in human freedom, but this is a mistake. By removing the ability to credibly commit to a long-term relationship, unilateral divorce prevents couples from reaching a mutually-beneficial bargain that greatly assists in the raising of children. Without forced marriage, which has never been part of the Western tradition, unilateral divorce actually removes an important choice. In the words of Robert Rowthorn (1999):

To get married is no longer such a major commitment and no longer offers the degree of security which it once did, since divorce is now relatively easy and the responsibilities and rights of the married and the unmarried are increasingly similar. These developments are often presented as an advance in human freedom since they allow individuals to exit unilaterally from unhappy relationships at minimum cost to themselves and with minimum delay. However, this is a one-sided view, since it ignores the benefits and freedoms associated with trust and security. The fact that individuals can now exit easily, and unilaterally, from a relationship makes it difficult for couples to make credible commitments to each other. They can promise anything they want, but most of these promises are no longer legally enforceable, and many are undermined by social policies which reward those who break their promises.

Because it no longer guarantees security (or anything else), marriage is much less useful and therefore less appealing.

The welfare state

Given that women will do most childrearing and that men are more productive, most societies will involve some sort of resource transfer from men to women. As with all redistribution, this would seem to carry the risk of punishing work. Yet in the case of marriage, it doesn’t. A man provides for his wife and children, who he has some legal and social claim on, as well as an obvious genetic interest in. The welfare state changes this.

Women spend longer in education, retire earlier, live longer, receive more in benefits and earn less. Source.

Rather than merely supporting their own wife and children, men are expected to support women to whom they have no relation, and from whom they can claim nothing in return. Not only is this much less motivating, but it also removes a major incentive for women to marry in the first place. The state can simply extract a potential husband’s wealth and transfer it to her, no marriage required.

In the long run, this is bad for women too. Even if they can raise children alone with support from the state, very few women want to do this. And by making marriage less attractive, the welfare state misaligns the short-term financial incentives of young women with their long-term social incentives as potential mothers.

The Western solution: a brief summary

If the Western European marriage pattern represented a cooperate/cooperate solution to the prisoner’s dilemma of human reproduction, our current dispensation represents an attempt to force a cooperate/defect equilibrium on men.

Men are expected to spend their lives working for women and children that are not their own. They can be ejected from a marriage at any time, for any reason, and by default will lose their children in the process. It is little wonder that, as in the Roman Empire, “marriage became unfashionable, especially among the men—but perhaps it would be more just to say that marriage on these terms was despised, for there seemed to be few advantages to be gained, many to be lost” (Unwin 1934). Conservative commentators often lament this attitude and blame influencers like Andrew Tate, but they confuse the cause for the effect. Men didn’t suddenly change their attitude towards marriage; marriage itself changed and men’s attitudes have slowly adjusted to the new reality. Browbeating and hectoring (“man up!”) cannot replace the old incentives.

Consequences of cooperate/defect

Men have the power to collectively force women into a disadvantageous marriage contract where they give up much in exchange for little (although Western men did not do this historically). But the reverse is not true. Men always have the option to not marry – and the alteration of marriage norms has made marriage less attractive for women, too, with predictable results.

It takes generations to see these effects in full. Not only are we the product of millennia of selection for marriageability; social norms are sticky. At first, men see that their fathers worked hard to get married and that their older acquaintances are doing the same, and imitate them. Women aspire to marry as their mothers did. Even when the law has changed, the norms do not immediately disappear. But they get weaker every generation. People see that marriage no longer offers stability. They see their peers and parents ruined by divorce. They see that they can get the economic and sexual benefits of marriage without giving up options. And the old norms erode.

This chart provides a good illustration of what I mean. When the incentives changed, marriage collapsed among blacks (who are not the product of millennia of selection for Western marriage) almost overnight. In whites, it has lasted longer, but each generation is less marriage-minded than the last. Everyone is responding to the new incentives, but not at the same rate. Source.

But let’s say that despite the hurdles you’ve found yourself a stable marriage with someone sensible enough not to divorce you over minor problems, and you’re even confident your own children could do the same. Why care?

Work

For the young men, that were most able and fit for labour and service, did repine that they should spend their time and strength to work for other men’s wives and children without any recompense. (Governor William Bradford on the failure of the Pilgrim’s experiment in communal agriculture.)

It’s common knowledge that married men earn more money than single men or women.

The gender wage gap is really a gap between married men and everyone else. Source.

It’s sometimes claimed that these higher earnings come from men who get married being intrinsically more productive (intelligent, conscientious and so on), rather than from marriage itself making men more productive. This is only partly true. Comparing cross-sectional to fixed-effects results shows that “selection on the basis of fixed unobservable characteristics accounts for less than twenty percent of the observed wage premium” (Korenman and Neumark 1991).

Incidentally, the “marriage premium”, a phrase used in the literature, is bad framing. Married men are not being gifted more money by competitive companies and there is no extra-market force such as affirmative action to benefit them. Husbands earn more because they are more productive. The prime beneficiaries of this are not the husbands, but their wives, their children and society at large.

The decline of marriage goes hand-in-hand with male economic decline. There is a vicious cycle: without the corresponding sexual and genetic rewards, men have less incentive to get ahead, and by virtue of having less money become less attractive. Source.

The post-60s settlement attempts to force men to transfer resources to women via the welfare state and child support. But as the Soviets discovered, it’s very difficult to get men to work to the best of their abilities through coercion alone.

Young men are increasingly dropping out. And why not? Source.

Without marriage, the state loses its taxpayers and society loses the men who make it work.

Crime

Rather than simply withhold their best efforts from a society that offers them little, men also have the option to actively attack it through crime. Crime can be thought of as a desperate, high-variance reproductive strategy. One might therefore expect that single men would be more criminal, and this is indeed correct. Within-individual studies (which control for selection effects) using Finland population registry data show that married or long-term cohabiting men are between 25% and 50% less likely to commit various sorts of crimes than their single counterparts.

I think the similarity between long-term cohabitation and marriage is important. Post-60s marriage is effectively just long-term cohabitation with a fancy title and the expectation of child support. See appendix. Source.

Social conservatives are wrong that broken families make children more criminal. But they are right that marriage reduces crime.

Fertility

People have begun to notice that the world as a whole is slipping below replacement fertility, and that the main centers of civilization have already been there for generations. Aged societies are weaker in war, less innovative in peace, and look to the past rather than the future. By providing a solution to the prisoner’s dilemma of human reproduction, marriage greatly boosts fertility, even today. This is true across the world, even in countries where the majority of births occur outside of wedlock.

What this chart is saying is that a hypothetical American woman who got married at 15 and remained married until age 50, who experienced the age-specific fertility of married women at each age, would have just over 4 children over the course of her life. American women rarely get married at 15 and stay married until 50, so this is not the same as saying that the average married couple in the US has 4 children. Source.

It’s not surprising that the shift from a cooperate/cooperate marriage system to a cooperate/defect one, and the attendant devastation of the institution itself, corresponds precisely to the end of the Baby Boom.

Share of married young adults over time. Source: The Upswing (Putnam 2020).

The most important thing marriage does is ensure society’s continuance. Hence it is relevant to everyone.

Happiness

One might hope to justify all of these changes by appealing to happiness. Take no-fault divorce. It’s not hard to find pre no-fault divorce horror stories and it’s easy to sympathize with the misery of being stuck with someone you hate but don’t have legal grounds to leave. Surely, then, with miserable marriages evaporating like dew, those that remain must be happier? And surely the unmarried, while still worse off than they would have been with a happy marriage, are better off single than miserable? Both answers appear to be “no”. Not only are there fewer marriages and far fewer stable marriages, but those that remain are less happy, not more.

Naively, you might expect the opposite. Less happy marriages are ended more easily, and since fewer marriages are formed, you might expect that unhappy marriages would be less likely to come into existence. So what’s going on? Notice that the biggest drop in the chart above is in the period from 1970 to 1985. This corresponds to the immediate spike in divorces following the introduction of no-fault divorce.

Relationship problems that seem insurmountable in the moment often (though not always) disappear with time, and sheer determination can smooth over a lot of bumps. But unilateral divorce gives an easy way out, which allows people to fixate on issues that might be resolvable, making marriages less pleasant.

Happiness by gender in the US. Marriage became far worse for men (women hit hardest). Source.

Where we’re headed

The shift from a cooperate/cooperate marriage system, where both men and women made sacrifices to gain the security required for childbearing, to a cooperate/defect one, where men are expected to uphold their end of the bargain in exchange for nothing, has failed. This is the legacy of second wave feminism.

Men are dropping out of work or burning things down, and both marriage and children are increasingly relics of the past. We are thereby moving towards a defect/defect system of the kind I described at the start. Men increasingly disdain the daily grind (bewildering public intellectuals, who fail to understand why men won’t respond to market signals). Birth rates continue to decline. And things slowly fall apart. In the event of war, few feel the urge to defend their country. We are in the early stages of the same sort of centuries-long collapse that many civilizations throughout history have suffered.

I am not saying this will happen. If the AI boffins crack superintelligence, neither marriage nor any other current aspect of human life will remain relevant for much longer. And forecasting social change ahead of time is notoriously difficult. Hence we could still draw back from the abyss. But if we do not change course, collapse is where we’re headed. An African future awaits.

Arctotherium is an anonymous writer interested in demographics and the future of civilization. You can find more of his writings at his blog Not With A Bang or at his Twitter.

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Appendix: marriage versus cohabitation

Modern marriage is effectively indistinguishable from cohabitation. The term survives out of cultural inertia, but doesn’t actually mean very much. This can make cross-country comparisons misleading because the ratio of marriage to cohabitation varies by country. I use the term “marriage” for post-70s marriage because I am American and the United States still has strong formal marriage norms for the middle class (see Charles Murray’s Coming Apart) and because formal marriage statistics are much more comprehensive and reliable than informal cohabitation statistics.

However, if you sum up marriage and long-term cohabitation, you will find the same trends apply even in countries where formal marriage norms have almost disappeared. In Finland, for example, first cohabitation has been slowly dropping since the 70s, while first marriage has collapsed. When combining the two, we see the same decline as in the US.

Timing of first cohabitation, first marriage, and first birth by birth year. Source.

And even in Finland, where 48% of children are born outside of wedlock, couples still prefer to wait until their relationship approximates the stability of marriage to have children. Which is why the timing of births in Finland closely approximates the timing of marriage rather than the timing of first partnership.

Timing of first partnerships, first births and first marriages in Finland. Source.

The same applies in Sweden, where cohabitation has only fallen slightly but formal marriage has declined substantially (meaning the combined measure has fallen). Here, fertility decline tracks the decline of marriage.

Fig. 3 — The decline of formal marriage in Sweden by regional origin. This has *not* been offset by rising cohabitation, which has slightly declined. Source.

Long-term cohabitation could be substituted for post-70s marriage everywhere in this article and the implications would still be valid. Pre-70s marriage offered legitimacy, social status, and a legal framework for stabilizing cohabitation. Post-70s marriage doesn’t. And that’s the problem.

Most forms of marriage also have other benefits, such as reducing sexual conflict by removing some partners from the list of legitimate prospects.

If we’d evolved with paternity tests, we’d probably be very different. But we didn’t, and our evolved psychology reflects uncertain paternity.

Economics plays a big role here. In an environment where women cannot support themselves and their children, such as Ice Age Eurasia or regions with intensive, plow-based agriculture like preindustrial Europe or China, this number is often less than one for the average man. In environments where women can support themselves and their children, this number can be arbitrarily high.

It goes without saying that “attractive” does not mean “prosocial”.

Joseph Henrich believes that monogamy is a group-selected cultural practice. Monogamy versus polygamy is arguably the most important divide in marriage, but is largely beyond the scope of this article, since polygamy has never been widespread in the West. See also my Review of Sex and Culture.

There are a bunch of other interesting features and effects of the Western European Marriage Pattern that are not relevant here. See my articles on The Western European Marriage Pattern or The Weirdest People in the World.

As opposed to between the parents or clans as in many societies, or theft in the case of groups that acquire wives via bridenapping, slavery, or conquest.