Dr. Hughes, a professor in the Department of Sociology at the University of CT, wrote in article in which he used the Prisoner’s Dilemma to explain Western society’s general practice of monogamy. He demonstrates the prisoner’s dilemma game and societal equilibria for three types of monogamy: patriarchal polygyny, loose patriarchal monogamy, and strict monogamy (the form of monogamy generally accepted and found throughout the US and Europe). The article can be found at: http://hackvan.com/pub/stig/life/Monogamy-as-Prisoners-Dilemma.html

Many anthropologists believe that monogamy is not actually a “natural” behavior for humans. We are one of few species to practice monogamy. Yet monogamy has been the reigning keeper of western relationships for as long as we can remember, with non-monogamous practices continuing to decline. Dr. Hughes reveals that as we see more gender equality we continue to see a decline in non-monogamy and the pay-offs for infidelity become harsher.  The US and most western societies are classified as strict monogamy societies. Under strict monogamy infidelity is likely to be reported (whether it be to a church, community, or even the law in divorce and custody cases).

Essentially, the loyal partner has more power over a partner practicing infidelity. Just as in the prisoner’s dilemma game where the confessor has more power over the non-confessor.

Hughes demonstrates the payoffs for fidelity and infidelity within a strictly monogamous society using the Prisoner’s Dilemma game where the Nash Equilibrium is (monogamy, monogamy) or (1,1):

The game uses several assumptions:

 (1) The value of a relationship can be summed up in one measure, a unitary dimension of demand.

(2) The value of a relationship is reduced if a partner is not loyal, bc of humanities general inclination towards jealousy or uncertanity. With 1 being a satisfactory whole and <1 being an unsatisfactory fraction. The relationship is not reduced to 0 because a libertine (a libertine classifying any human in western society) might recieve some satisfaction knowing that their partner’s infidelity frees them to engage in infidelity as well.

(3) The utility of a monogamous relationship is equivalent for everyone. 

(4) Sexual contracts are reciprocal and subject to the same constraints.

The majority of American couples see the risks of covert infidelity, in disease, partners’ discovery, etc. to be inferior to its benefits. Even affairs rarely continue for any length of time without resolving into monogamy with one partner or the other (Hughes, 1992). Despite some infidelity the pay off matrix is still that seen in the above figure (figure three) with mutual monogamy equal to an equilibrium.

Even the minority of people who have come to accept infidelity in their partner is trapped in monogamy by a prisoner’s dilemma. As long as they are each pursuing the best outcome for themselves… non-monogamous arrangements collapse, and society is forced to revert to monogamy.

 Huges writes that “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.”  Although this has been seen in some strict mormon communities in the past, the NE of monogamy has led these communities to dissolve.

Despite species natural tendencies toward polygamy, monogamy remains a natural Nash Equilibrium for Western society and it appears likely to remain that way unless society completely reverts its relationship norms and values. Whether we are forced into fidelity through the prisoner’s dilemma or whether are values and norms created the prisoner’s dilemma in the first place, is debatable.