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Monogamy as a Prisoner’s Dilemma: Non-Monogamy as a Collective Action Problem
J. Hughes   Dec 26, 1990  

An essay on the difficulties of establishing stable polyamorous threesomes.


Since I wrote this thought piece in graduate school in 1990, and posted it on the Web in 1992 for comment, I have received dozens of emails from polyamorous people on the Net. The majority take issue with my assertion that polyamory has not been successful as a subculture, putting forth their personal experience and the burgeoning polyamorous subculture as counter-evidence.

One important complaint is that I define success as equivalent to the longevity of the plural relationship. Certainly one could alternatively define polyamory as a success if the subculture continues to grow, although all the participants are constantly, happily, changing their configurations of partners. For monogamy, this would imply that monogamy is a success since most people are monogamous until they switch partners, even if they do so every week. On the other hand, serial monogamy is clearly more problematic for those who hold with romantic ideas of romantic love, that binds (only) two souls forever, than serial polyamory is for those who are rejecting possessive relationships. In other words, my definition of success may be a foreign and inappropriate one for the polyamorous subculture.

A second problem with this paper is my rigid set of assumptions about the nature of human preferences, and the rewards of relationships. I conclude that polyamorous arrangements are impossible to maintain if participants can defect to receive a monogamous partner’s undistracted attention. This assumes that the average polyamorous participant finds the attentions of one undistracted person more satisfying than the divided attentions of two (or more). This is more likely to describe people who find themselves involved in a polyamorous situation by accident, rather than those who intentionally choose polyamory. It is almost tautological to say that polyamory will be stable for those for whom the various rewards of the polyamorous arrangement are superior to the rewards of monogamy. If there are enough people with polyamorous preferences, or who discover that polyamory is superior for them, then the subculture can establish a foothold. 

A second conclusion of this thinkpiece is that, under these limited assumptions, polyamory would be a stable subcultural option if defection were impossible or punished. For instance, a community where all potential partners are married would make the polyamorous arrangement of mate-swapping relatively stable. Unfortunately for contemporary mate-swappers, their community would punish their deviance, making these arrangements covert, less attractive and rare. 

I point to the historic success of the Oneida community, with its strictly controlled polyamory, as an example of a community which explicitly planned and rewarded non-monogamy, and punished monogamy. Oneida became one of the most successful of 19th century communes, while the "free love" communal experiments were the shortest lived. Most contemporary polyamorous people would find the idea of such a coercive arrangement unattractive, however.

The question is whether the polyamorous subculture can become stable solely because of the rewards of polyamory, without also being able to effectively punish defection as the Oneidans did. If the polyamorous subcultures becomes big enough, it might achieve the "tipping point" where polyfidelitous norms (and their associated informal punishments for violation) are self-sustaining. But there are steps that could be taken to help the process along.

As defenders of the traditional family correctly argue, the institution of marriage, and the relative difficulty of divorce, help convince people to persist in relationships that might otherwise dissolve. The benefits of legal marriage (health insurance, etc.) are a reward for maintaining a bond, and the difficulties of divorce (legal fees, division of assets, alimony, child support) are a punishment for defection. This is one reason that polyamorous people should be concerned with reforming marriage laws to recognize gay and multiple partnering. 

The many responses from the net have led me to a new line of research, setting up artificial life models to test different propositions about the interaction of preferences and opportunities in predicting the success or failure of monogamy/polyamory. Any thoughts are, therefore, very welcome for my further modeling.


Many ethologists and anthropologists now believe that monogamy is not "natural" behavior for humans. Yet non-monogamous practices have declined with modernization, and the occasional attempts to reintroduce non-monogamous practices in the contemporary West have failed. This paper uses the perspective of rational choice theory and strategic interaction to examine three known equilibrium social states: patriarchal polygyny, loose patriarchal monogamy, and strict monogamy.

The paper then turns to contemporary liberal egalitarian society and examines strategic interactions among sexual consumers with strong non-monogamous preferences. Non-monogamous experimenters are shown to be in a prisoner’s dilemma; if all pursue utility-maximizing strategies, their resulting collective satisfaction is less than if they remain monogamous.

Finally, while collective action to enforce non-monogamous norms can resolve the prisoner’s dilemma, and make non-monogamy a sustainable sub-culture, the enforcement of these norms are incompatible with the libertinism that motivates egalitarian non-monogamy in the first place.

Thanks to James Coleman and Norman Braun for their comments on this paper, though I was unable to integrate much of the more sophisticated bits into this essay.


Sexual radicals have asked "why monogamy?" many times, but only recently have ethologists and anthropologists re-examined the assumption that monogamy is a "natural" condition. Ethologists now believe that only 1-2% of all species may be monogamous (Angier, 1990). None of the simian species are strictly monogamous; our closest relatives, the chimpanzees, practice a form of group marriage. Among the 849 human societies examined by the anthropologist Murdock (1967), the vast majority (83%) practiced polygyny, men having more than one wife; monogamy was characteristic of only 16% of the societies. Even in societies that are technically monogamous there is often a good deal of both tolerated and covert infidelity.
Observers attribute human non-monogamous sexuality to two basic motivations. First, many ethologists believe that there is a sociobiological imperative to have as many sexual partners as possible. Non-monogamy is reproductively savvy for males in order to spread their genes, and for females in order to improve the hardiness and genetic variety of their off-spring. Secondly, most humans find a variety of sexual partners pleasurable, whether reproductively savvy or not. All other things being equal, some form of non-monogamous equilibria would seem to be natural.
There are basically three known kinship systems known to have been widespread and equilibrious for some time: (1) patriarchal polygyny, (2) patriarchal monogamy, with widespread male infidelity, and (3) strict monogamy. After examining the strategic situations involved in each of these equilibria I will examine whether a non-monogamous equilibria is possible in contemporary liberal egalitarian society.

Patriarchal Polygyny

Patriarchal polygyny isn’t really a strategic interaction between sexual partners, but rather an economic decision for the man. Under polygyny, wives had no real decision to make but to accept their fates, join a religious order (if available), or kill themselves (the choice of exit). On the other hand, we can assume that there was some possibility for female infidelity. But we can also assume that female infidelity was extremely difficult to conceal in tribal polygynous societies, and that its penalties were extremely heavy.

Both genders also face the costs and rewards of internalized norms. In this situation we can assume that male internalized rewards are colinear with number of wives, while female internalized norms are colinear with the strength of the potential punishment.

Male payoff = (Demand for w wives) + (prestige*w wives) + (Labor value of w wives) - (cost of w wives) = f(w) > 1
Female payoff = (Demand for infidelity) - (risk of discovery*strength of punishment) - (internalized sanction) =

The consequent pay-off matrix for the wives and husbands would be something like this:

If the risks of discovery were lessened by circumstances in some societies, such as the sheikh being away from the harem for an extended period, then the wives might risk infidelity. If we further postulate a principal of strength in numbers, or gender solidarity, among the wives, then we might find that infidelity would be more common among the polygynous households. But gender solidarity faces the collective action difficulty: what if one of the more ambitious wives reports other wives’ infidelities to the sheikh? If the wives were mutually suspicious we might find, instead, that infidelity was more common among the relatively autonomous, single wives.

Loose Patriarchal Monogamy

Anthropology still debates the origin of monogamy. Gary Becker has suggested that monogamy may have arisen as a means for poor and unattractive men to ensure that they have access to wives.

Since monogamy appeared, however, monogamous societies have varied between relative libertinism and puritanism. Under the looser, libertine patriarchal monogamy, the community, church and state have generally not approved of extra-marital relationships, but not strictly enforced these norms for men. Often there is also an implicit prestige for sexually promiscuous men.

But loose monogamy differs from polygamy in two ways. First, the internalized rewards of non-monogamy (macho prestige) are muted by its official prohibition. Second, men can only exploit the labor power of their one wife, while they must continue to provide material gifts to their mistresses. Mistresses may cost as much to maintain as wives, and are surely more difficult to exploit the labor of. Thus, even loosely observed monogamy sharply reduces the incentive to seek additional, simultaneous mistresses, compared to polygyny.

Male payoff = (Demand for m mistresses) + (m mistresses*prestige)  - (cost of m mistresses) = f(m) > 1

Female payoff = (Demand for infidelity) - (risk*punishment) =

< 1


In general, men will choose to cheat in such a society, to the extent that they can afford the extra mistresses, and women will not cheat, to the extent that they face punishment for doing so.

Strict Monogamy

Under puritanism, however, infidelity is more likely to be reported, and if reported severely punished. (Of course, the punishment is still usually more severe for women than men.) In addition, we can assume that the internalization of norms in the puritanical phase is more thorough, displacing some or all of the covert prestige of male promiscuity with guilt and fear. The result is stricter adherence to fidelity by men, and a monogamous equilibrium.

Since the Enlightenment and Industrial Revolution, the growing equality of women has also contributed to the more strict adherence to monogamy. On the one hand, men and women have gradually been treated more equally before the law, and in defending women against domestic battering. On the other hand, women have gained increasing choice in husbands, through "love marriage" and divorce, and increasing economic independence from their husbands. Women thus have both the powers of turning their husbands in to the church or community, if not the law in divorce proceedings, and the power of "exit," as sanctions they can levy against philandering husbands. This female empowerment would create a trend towards either greater male fidelity, or at least greater male care in hiding their infidelity, which would increase its cost, and decrease its demand.


Male payoff = (Demand curve for w women) - (risk*punishment) - (internalized sanction)

Female payoff = (Demand curve for p partners) - (risk*punishment) - (internalized sanction)


Further Simplifying Assumptions

In order to illustrate the prisoner’s dilemma of monogamy I need make a couple more simplifying assumptions:

(1) The first assumption is that the value of a relationship can be summed up in one measure, or rather, that relationships fulfill a unitary dimension of demand. In fact, one of the major rationales for non-monogamy is that different people can fulfill different parts of one’s needs, in ways that can’t be compared or traded off. For instance, in Spike Lee’s She’s Gotta Have It, the protagonist has three lovers, one for fun, one for romance, and one for financial stability. But estimating a model for one utility is difficult enough.

(2) A second assumption is that the utility of a monogamous relationship is equivalent for everyone. In reality, people assign different amounts of utility to different combinations of commitment, shared activity and intimacy with different numbers of people. Some people can substitute non-relationship utility for relationship utility, finding their ideal combination to be half a relationship and a full-time hobby or career. We will ignores all these variations and assign everyone a utility of 1.0 for a monogamous relationship.

(3) The third assumption is that while the value of the first full-time relationship is 1.0, each additional simultaneous relationship has declining marginal value. In other words, you are less than ten-times happier when engaged in ten simultaneous relationships.

Obviously, the rate of declining marginal utility is very different for different people. For the voluntarily celibate, the value of even one relationship is less than its cost in time and resources, perhaps because they value the rewards of prayer, art, or politics more. And for some rare individuals, there may even be a multiplier effect from non-monogamy, though this would still face an eventual constraint; philanderers may be three times happier with one mistress, but thirty times happier with ten mistresses. The risks of sexually transmitted disease have added an additional degree of decline for the value of additional relationships.

In my model below I will simply assume that a second relationship is worth only 0.75. With relationships with two single others one receives 1.75. The model will make the simplifying assumption that two simultaneous relationships are as many as these experimenters can deal with.

(4) A fourth assumption is that sexual contracts are reciprocal. Men and women are now subject to the same constraints. Even with relatively gender equality, there are today many situations where powerful partners can enforce fidelity on dependent lovers while they philander. Men and women of great wealth, power or charisma are often able to convince their lovers to remain faithful even if the advantaged person is not. This model, however, will only discuss situations where all partners either agree to monogamy, or consent to one anothers’ non-monogamy.

(5) A fifth assumption is that the value of a relationship is reduced if the partner is having another relationship. This is clearly true for most people, because of jealousy and uncertainty. For many, a partner’s infidelity reduces the value of a relationship to zero.

But the following model is primarily concerned with libertine sexual experimenters, who do not suffer from jealousy or uncertainty, and receive some degree of satisfaction at the idea that their partner’s infidelity frees them to reciprocally engage in infidelity themselves. They do suffer from a loss of some of their partner’s time and attention however. The net loss of utility for a libertine with an unfaithful partner is thus assumed to be only a third, rather than 100%.

(6) A sixth assumption is that there is no declining utility of a relationship over time. Nobody gets bored. Nor does utility increase with time: nobody becomes attached.

Monogamous Majority and Non-Monogamous Minority in Liberal Society

While about a third of married Americans have experimented with affairs, the majority of American couples see the risks of covert infidelity, in disease, partners’ discovery, and so on, to be inferior to its potential benefits. Even affairs rarely continue for any length of time without resolving into monogamy with one partner or the other. The pay-off matrix for most Americans is still essentially that of Figure Three, with monogamy as equilibrium.

But a small minority of every liberal democratic Western society has strongly desired sexual experimentation, and rejected monogamous norms and values. Whether their desires result from anti-monogamous socialization, or from the absence of the suppressive effects of pro-monogamous socialization, they see the rewards of not only covert infidelity, but even reciprocal non-monogamy as greater than its costs. The following model attempts to model the pay-offs that such non-monogamous experimenters might face.

At the beginning, each partner faces a choice. They can choose monogamy, or choose to also engage in an affair with some single person. The supply of single people is a critical variable which we will address later, but for now we assume that there is a ready supply of single people who are also interested in non-monogamous relationships. For these non-monogamous experimenters A and B, their pay-offs having affairs are always better than the pay-offs remaining in monogamy.



This takes us from box 1 to box 2, where the central couple, A and B, are both having affairs with single others, C and D. But C and D are strategic actors as well, and attempting to maximize their utility. They face the same pay-off matrix as A and B do, under which it never makes sense to have just one lover. So C and D also seek out single lovers, E and F. This takes us from box 2 to box 3.

Again, E and F are rational actors, and they seek out other lovers, moving us to box 4. If actors A through F, in this sub-culture of non-monogamous experimenters, each have two lovers, then A through F are receiving 1.17, which is greater satisfaction than they got under monogamy (1.0).

The first problem is that the people at the ends of these strings are only receiving .66, which is less than they could get in a monogamous relationship. But less us assume an infinitely expanding chain for the moment, and turn to the second problem: everyone receiving 1.17 is aware that they could be receiving 1.75 if their lovers did not have other lovers, and they were the center of two undivided attentions. For A, this situation is box 5A, and for B, 5B. Or at least actors could achieve 1.51, if they only had to share one of their lovers (the "6" boxes). This is where the conditions of the environment become critical.

In order to defect from general non-monogamy (box 4) to a more privileged non-monogamy (boxes 5 or 6), each player must calculate the likelihood of their being able to move from the position they are in to the advantaged position. If there is 100% certainty that they can break off relations with one or both of their lovers and replace them with single lovers, then they will do so, achieving either 1.51 or 1.75. In the case of B, for instance, this would mean moving from box 4, to boxes 2, 5B, or 6B.

The complexity enters here. If everyone has a good chance of finding single others who are willing to engage in non-monogamous relations, then everyone’s best strategy is to break off relations with involved others and seek out single others. But if everyone breaks off with involved others, then the environment changes. When no one will maintain a relationship with someone involved with someone else, the greatest number of sustainable partners is one, i.e. monogamy. Neither A nor B can find single others who are willing to be involved with them, since they are already involved with each other, and therefore their subculture reverts eventually to an equilibrium around box 1, monogamy.

This is the prisoner’s dilemma. If everybody takes two lovers, then everybody gets 1.17 (rather than 0 for the singles, and 1.0 for the monogamous). But since everybody can do better if they are the only one with another lover, then nobody can have two lovers, and everybody only gets 1.0.


Building Non-Monogamous Equilibria in Liberal Society

There are several "natural" and several "voluntary"

solutions to this prisoner’s dilemma. One "natural" or environmental solution results if it is impossible to find singles willing to get involved with a non-monogamous situation. If there is little likelihood of finding two single others to defect to, a set of non-monogamists who have reached box 4 would have no incentive to leave it, since the only real alternatives are monogamy (1.0) or general non-monogamy (1.17).


This set of successful non-monogamists would need to find a solution for the ends of their relationship chain, those receiving less than monogamous satisfaction [.66 - 1.51 - 1.17 - 1.17 - 1.51 - .66]. This arrangement would slowly unravel. One solution is to close the circle at the ends, providing a uniform 1.17. Another solution is to find individuals who are uniquely attracted to this limited commitment, such as those who do not have the time or ability to commit to a monogamous or multiple relationships.

Spouse-Swapping Equilibrium

We might imagine a sleepy suburban 1950s Peyton Place, where 99% of all the adults are married. In this situation one’s only choice is whether to have an extramarital affair with a married person or not. Column 2 and 3, and Rows 2 and 3, which presume the existence of single people, are simply not available. For the potential non-monogamous experimenter subset of the married couples, the choice is obvious: spouse-swapping, or a small closed circle. As long as the chance of finding singles to connect with is close to 0%, the spouse-swapping arrangement will be equilibrious.

Voluntary agreements can also structure equilibrious non-monogamy. Fear of disease may cause actors to want to set clear boundaries on their partners’ contacts: no one may sleep with anyone outside the group. Long-term friendships may have established trust between the partners, making contacts with others more costly and risky. The participants may have arrived at these voluntary agreements through learning from previous affairs that closed circles were the only successful arrangement.

These agreements are probably most common among a closed circle of three, a menage a trois.

The problem is finding partners who will agree to the concept of menage a trois or group marriage, and to enforcing the contract. The State does its best to discourage such contracts, with laws against polygamy and bigamy, and refusing custody and other rights to such non-traditional families. The community is usually even less help, heaping buckets of scorn on the sinners, and rewarding partners who come to their senses and leave the arrangement.

Achieving Critical Mass

Therefore, those with non-monogamous preferences must organize for collective action, both in order to enforce collective norms governing non-monogamy, and in order to throw off the norms that discourage non-monogamy. Sexual deviance is a local "public good," requiring collective action. From the gnostic "free-love" rebellions of the Middle Ages to the Stonewall riots, the institutionalization of sexual deviance has required the gathering and organization of sexual radicals, who then made deviance safe for less committed experimenters.

If the initial risk-takers are successful, and survive long enough, they can attain "critical mass." The critical mass number is the "tipping point" at which the external and internalized inhibitions reduce to the point that benefits for group members exceed costs, and membership in the sub-culture is self-sustaining (Schelling, 1979: 100-110). My argument above is that, while many sexual radicals have tried to create a self-sustaining non-monogamous subculture, they were never able to achieve critical mass or sufficient agreement as to what the rules should be for "normative closure."

Occasionally one individual has been able to afford to provide the public good by themselves. For instance, the legalization of divorce was a public good provided by King Henry VIII, making British divorcees a "privileged group" in Olsonian terms (Hardin, 1979:35). Though all British desirous of divorces benefited, only Henry VIII could afford to legalize divorce.

Without such a hegemon, those with non-monogamous preferences are a scattered "latent group," awaiting entrepreneurs willing to risk organizing costs in order to reap later rewards from leadership. For instance, the founder of the Mormon Church, Joseph Smith, can be seen as a public goods entrepreneur, organizing to re-create polygyny, though it ultimately cost his life, and federal troops massed on Utah’s border undid his work.

But Mormon polygyny, which was strictly patriarchal, does not really address the possibility of a non-monogamous equilibria in liberal, egalitarian society. Today sexual alternatives face even fewer external and internalized sanctions than they did in the 19th century, and a century of Western sexual libertines have experimented with those alternatives. Yet only one large community is known to have developed a system of egalitarian non-monogamy that lasted more than a few years: the Oneida commune of up-state New York (1837-1879). Oneida may illustrate some of the ironic complexities of attempting contemporary free love.

Oneida was founded by John Humphrey Noyes, a Perfectionist minister who preached "Bible Communism" and rejected both monogamy and polygamy as forms of ownership to be forsaken by the saved. But he also fervently rejected the anarchistic "free love" practices of his contemporary sexual radicals, such as the Fourierists, Owenites and spiritualist feminists who advocated liberalized divorce and promiscuity, and insisted that "complex marriage" could only be practiced within a community under spiritual discipline.

In Oneida, if a man or woman desired a sexual liaison with another member they could petition a community elder to carry a message to the party in question; they were forbidden to ask other communards directly. If the other consented, which was not certain, then the couple could meet several times, but no more; if they developed any feelings of attachment they were immediately separated. If they violated the community’s rules, they could be expelled. For the first 17 years of Oneida’s existence, men were forbidden to ejaculate, as a contraceptive method and an aid to female sexual pleasure: there were no pregnancies recorded for that period. At its largest, the complex marriage system included about 300 people, and it lasted more than thirty years.

Rosabeth Moss Kanter (1972), in her statistical study of one hundred 19th century communes, correlated sexual systems with the longevity of the commune, and concluded that the Oneida-type complex marriage systems, and the celibacy practiced by the Shakers, were correlated with commune longevity. At the other extreme were communes like Berlin Heights that attempted to institute "free love"

without strong community controls; these communities were invariably short-lived. In the end, Oneida’s disbanding had nothing to do with dissatisfaction with the complex marriage system, but rather grew out of the secularism of the second and third generation of the community, who were no longer prepared to make the other sacrifices of communal life; the combination of strong preferences for sexual variety and a strong commitment to religious authority proved unique to just one cohort.

Much of this account are elaborate truisms: if you punish some people heavily for doing something pleasurable, and others not at all, it is likely that the first group won’t do it while the second group does. But in the elaboration of the contemporary matrix for non-monogamy we do discover a counter-intuitive result: even those with strong preferences for non-monogamy, who have freed themselves from internal and external costs from such deviance, and are committed to tolerating non-monogamy in their partners, even this minority is trapped in monogamy by a prisoner’s dilemma. As long these libertines are each pursuing the best situation for themselves, all the non-monogamous arrangements collapse, and the libertines are forced to revert to monogamy.

To escape from this prisoner’s dilemma the potentially non-monogamous must organize, submit to a new set of enforced norms, and create a non-monogamous community of sufficient size to achieve closure. But the same libertarian impulses that lead most contemporary sex radicals to pursue "free love," repel them from the Oneida-style authoritarianism which makes it viable.

Consequently, we now have many anecdotes of "free love" veterans of the Sixties back-sliding to a revisionist advocacy of monogamy. Public disapproval of adultery among Americans has risen from 80% to 85% since the 1970s, while disapproval of other forms of sexual deviance, such as homosexuality or premarital sex, has declined. AIDS has, of course, added another whole dimension to the "costs" of non-monogamy. Natural or not, monogamy appears likely to remain with us for some time.



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James Hughes Ph.D., the Executive Director of the Institute for Ethics and Emerging Technologies, is a bioethicist and sociologist who serves as the Associate Provost for Institutional Research, Assessment and Planning for the University of Massachusetts Boston. He is author of Citizen Cyborg and is working on a second book tentatively titled Cyborg Buddha. From 1999-2011 he produced the syndicated weekly radio program, Changesurfer Radio. (Subscribe to the J. Hughes RSS feed)

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