By Albert Gustafson
Laws against polygamy are predicated on the idea that traditional families, the center of which is monogamy, create positive externalities; whereas non-traditional families create negative externalities. But like any law, their goodness as laws turns on their robustness and efficiency.
This problem can be understood in Pigouvian terms, since polygamy produces social costs higher than private costs and monogamy produces social benefits higher than private benefits. Specifically, polygamy tends to lead to less family stability and more familial strife. Because polygamy is usually polygynous and not polyandrous, it also causes more violence amongst the men of the community, as they compete more furiously over smaller pool of mates. On the other hand, monogamy produces healthier children. The wedge between the social costs/benefits and private costs/benefits of polygamous and traditional marriages may be especially large because healthy, productive children benefit society after the death of the parents.
But the society that those healthy children benefit is also made up of different people than the people enforcing the laws. So what interest does the law have in healthy children? A commitment to healthy child-rearing requires a forward-looking charity towards humanity that does not seem to have much to do with “self-interest,” except insofar as people are “interested” in being virtuous.
People’s motivations for solving the problem of future social cost are difficult to understand with economic tools, but there are other, non-child-oriented social costs in polygamy. These costs have mainly to do with increased competitiveness in the search for mates and the violence that can motivate. Solving this problem efficiently is simple: punish fighting over mates. But most legal codes already include laws that punish fighting of all kinds – a robust solution. That means the remaining social costs of polygamy have to do with child-rearing and are not costs to those attempting to solve them. Yet solve they do, meaning that the law’s prohibition of polygamy has primarily to do with the costs and benefits of bad and good child-rearing.
The textbook model for solving externalities and emphasizes efficient, Pigouvian prescriptions. For instance, if the benefits of healthy child-rearing are not fully realized by parents, a Pigouvian subsidy on positive behaviors, like spending time with one’s children, would be an efficient solution; likewise, if parents do not fully bear the costs of poor parenting, a Pigouvian fine on negative behaviors, like favoritism, would be efficient. They specifically solve the parental problems with polygamy. But the efficiency of these solutions to the social costs of poor family structures comes at the price of being unenforceable and intrusive.
A more robust solution would attempt to understand an overarching principle of behavior that gives rise to social cost. In this case, polygamy is that behavior. Because polygamy laws resolve a variety of social costs without regulating specific actions, they have a robust quality. If the law viewed polygamy as a bad in itself, then the law might be both efficient and robust.
However, the robustness of polygamy laws comes with drawbacks. Since their primary function is to prevent specific actions, like fighting over mates or playing favorites with children, without directly addressing those actions, polygamy laws cannot entirely succeed in fulfilling their goals to reduce the costs created by bad parenting. Monogamists can be bad parents, too.
The real laws against polygamy also suffer from the inefficiency that they simply ban polygamy altogether. As with pollution, there is presumably a “socially optimum” level of polygamy that the market could discover, if transaction costs were low enough to allow polygamists to negotiate with those who bear the costs of polygamy. Yet as before, the society to whom the costs and benefits of polygamy accrue is in the future, making negotiation impossible.
Understanding how societies rationalize laws against polygamy, then, requires returning the virtue language from before. “Now faith is the assurance of things hoped for, the conviction of things not seen.” Rather than monetary negotiation, most societies have viewed their obligation to future generations as repaying their parents for the positive externalities they already received from their parents’ good stewardship; likewise, parents expect their own good stewardship to beget good stewardship in the future. Because these cultural norms determine the gamut of specific child-rearing practices and family structure, they are an efficient and robust solution to the problems anti-polygamy laws attempt to solve. The laws are robust, but norms are simultaneously robust and efficient.
 “Robustness” is here defined as a law’s adaptability to a variety of legal disputes; “efficiency” is defined as a law’s thoroughness in dealing with one specific issue.
 Bergmann, Barbara. “Becker’s theory of the family: Preposterous conclusions.” Feminist economics 1, no. 1 (1995): 141-150.
 Smith, George D. “Nauvoo Roots of Mormon Polygamy, 1841–46: A Preliminary Demographic Report.” Dialogue: A Journal of Mormon Thought 27, no. 1 (1994): 1-36.