However, there is also the possibility for competition and exploitation in marriage, both in the formation state and at any point after formation. We would expect humans, as embodied reproductive strategies, to have emotional, physical, and cultural means of dealing with competition and the risk of exploitation, but allowing for the formation and maintenance of marriages under auspicious circumstances.
The environments in which humans adapted (EEAs) include other co-evolving organisms, such as dogs, parasites, and prey species. Similarly, human EEAs include co-evolving cultures. If the culture in which we evolved suddenly changes, it may wreak as much havoc as if the species we evolved to eat (or which evolved to eat us) suddenly disappeared.
I believe the norms surrounding marriage are such cultural elements. We evolved in cultures with certain norms surrounding marriage; these varied between cultures, but not arbitrarily. Our culture has changed drastically - exponentially - in recent years, and the elements of culture relating to marriage have been some of those that have changed the most.
One way in which culture has supported marriage is through coercion: once married, societies enforced ongoing duties between the pair. Parties to a marriage were forced to make the best of their one marriage, as they likely would not get another shot. While this would force some to remain in bad, even abusive, marriages, it probably benefited most by encouraging couples to form and maintain good marriages. It's a sort of spike on the steering wheel. As generally occurs in human institutions, a few unlucky folks are thrown under the bus for the good of the other monkeys.
But culture has also supported marriage by providing norms for mate-finding and marriage practices that helped our ancestors maintain marriage communities that accomplished their purposes.
Parties must choose whether to form a marriage. Even if you're a woman in a nasty EEA and get sold by your parents to your 70-year-old uncle or whatever, you still must choose whether to cooperate or defect within the limits of your power. Not just at the inception, but throughout the marriage, each party constantly faces the choice to cooperate in the marriage (a good strategy if the marriage is a good one, as marriage has substantial health, fitness, and happiness rewards to offer) or defect (a good choice if the marriage appears to be a bad one for various reasons).
However, defection is not all-or-nothing. If things aren't great, a minor reduction in commitment ("shit test," you might say) might be more appropriate than an outright defection. But a reduction in commitment can either trigger increased commitment by one's partner, or trigger a counter-reduction. In the latter case, the marriage community spirals out of control into a defection cycle.
What are the ancestral behaviors that might promote a happy marriage? What cues might indicate a sinking ship? We'd all like to know that, and I don't pretend to be an expert on marital behavior. But what follows is my best guess as to some behaviors and triggers that either reinforce or erode a marriage community.
Behaviors that Reinforce Marital Harmony
- Nutrition sharing. Human couples seem to have evolved to form hunting/cooking partnerships. Division of labor by sex is on the list of human universals. Norms often treat men and women in possession of food differently, reflecting different sense of property rights based on the existence of a marriage community. Even if you don't buy the gender stuff, co-eating seems to be an extremely important ancestral method of reinforcing a pair bond.
- Regular, exclusive sex.
- A mutual mental commitment to indefinite future cooperation (being "all in")
- Mutual mental modeling from frequent interaction
- A 5:1 or greater positive:negative interaction ratio
- Co-sleeping (and co-living in general)
These behaviors were once common, even socially obligatory, between married couples. How common are these behaviors now?
What behaviors might trigger our evolved defection responses? For the most part, these are the mirror image of the above reinforcing behaviors:
- Failure to share nutrition, cook, or eat together
- Treatment of the marriage as a market relationship
- Lack of regular sex
- Sex outside the marriage
- Close friendships with members of the opposite sex who might threaten the relationship (opportunities to jump ship)
- Evidence of low mental commitment to future cooperation
- Crankiness (lower than 5:1 positive:negative interaction)
- Lack of interaction or mutual interest resulting in poor mental modeling
- Irregular co-sleeping or co-living
Even though modern couples may have solid reasons for engaging in the above triggering behaviors, it is not necessarily the case that we can control our ancestral responses to such behaviors.
A triggering behavior is likely to at minimum trigger a reduction in commitment from one's partner. If the response is not a reevaluation and recommitment from the "offending" partner, but rather no response or a negative response, an escalating cycle culminating in outright defection is likely.
The maintenance of the marital community - keeping the cooperative strategy operational, while preventing the defection strategy from being triggered, and certainly preventing it from triggering a defection cycle - is one job human culture used to do. It would be nice if modern independent, individual humans were able to take up the slack, but that is not a job our brains and bodies evolved to do. Those who manage to create lasting, happy marriages are fast becoming the minority. In addition to the broken-hearted marriage refugees themselves, the biggest losers are the children, brought into the world without their consent and denied their ancestral privilege of growing up in a functioning marital community.