MOST people believe that consenting adults should be free to do as they please, provided they do not cause unacceptable harm to others. The difficult question, of course, is what constitutes unacceptable harm. The debut on Sunday night of "Big Love," the new HBO series about a polygamous fictional family in Salt Lake City, has touched off renewed debate about this question.
Barb, Nicki and Margene, the three heroines of "Big Love," chose to marry Bill Henrickson, a successful businessman able to provide generously for their extended family. Mr. Henrickson chose to marry them. Should society outlaw such arrangements because they cause unacceptable harm to others? If so, who is harmed, exactly, and how? Economic theory, it turns out, has interesting things to say about these questions.
The traditional argument against plural marriage is that it harms women, particularly younger women who may be coerced to enter such marriages. Needless to say, society should prohibit forced participation in any marriage, whether plural or monogamous. But mature women who freely choose plural marriage reveal a preference for that arrangement. So if plural marriage harms women, the victims must be those who prefer monogamy.
It is easy to see how some of these women may be harmed. In a monogamous world, for example, Barb's first choice might have been to marry Bill, who would also have chosen to marry her. But with plural marriage permissible, Bill might prefer to marry not just Barb, but also Nicki and Margene. Barb would then have to choose between two lesser outcomes: a continued search or a plural marriage not to her liking.
Of course, the mere fact that allowing plural marriage may eliminate attractive options for some women does not imply that it imposes unacceptable harm on women generally. Suppose, for example, that if polygamy were legal, 10 percent of adult men would take an average of three wives apiece and that all remaining marriages would be monogamous. Among aspiring monogamists, there would then be nine men for every seven women. The law of supply and demand applies no less to social relationships than to ordinary commercial transactions. With an excess supply of men in the informal market for monogamous marriage partners, the terms of exchange would shift in favor of women. Wives would change fewer diapers, and their parents might even escape paying for weddings.
What about men? Here, too, plural marriage would clearly benefit some. After all, there are surely other men like Bill Henrickson of "Big Love" who would not only prefer multiple wives, but also be able to attract them.
But what about those who prefer monogamy? Permitting plural unions would, as noted, create an imbalance of men over women among monogamists. With so many formerly eligible women no longer available, the terms of exchange would turn sharply against men (as happened recently in China as a result of female infanticide). Many men would fail to marry at all.
In short, the logic of supply and demand turns the conventional wisdom about plural marriage on its head. If the arrangement harms others, the most likely victims are men, not women.
This conclusion is reinforced if we take account of the costly, and mutually offsetting, jockeying for position associated with men's attempts to win the attention of scarce women. The inherent wastefulness of such "positional arms races" is vividly illustrated by examples from nonhuman animal species.
The overwhelming majority of such species are polygynous, meaning that some males take more than one mate. Since having multiple mates is extremely advantageous in Darwinian terms, males typically battle one another ferociously for access to females. Size often decides these battles, so males tend to be considerably larger than females in polygynous species.
Some bull elephant seals, for example, are more than 20 feet long and weigh more than 6,000 pounds (about as much as a Lincoln Navigator), whereas females are typically less than 12 feet long and weigh about 1,500 pounds. Natural selection favored larger males because the winners of the long and bloody battles between males often command nearly exclusive sexual access to harems of more than 50 females.
But although being bigger than one's rivals is clearly advantageous for large seals, they are also less mobile and hence more vulnerable to sharks and other predators. Relative size, not absolute size, governs the outcomes of fights, so it would clearly be better if each male were only half as large. All fights would be resolved as before, yet all males would be less vulnerable to predators. Unfortunately, however, seals have no practical way of curtailing the arms race that makes them so big.
Permitting plural marriage in human societies would unleash competitive forces analogous to those we see in other species. With women in chronically short supply, men would face even more intense pressure than they do now to get ahead economically, to spend even longer hours honing their abs. More men would undergo cosmetic surgery. Expenditures on engagement rings would rise. Valentine's Day bouquets would be two dozen roses, not just a dozen. Yet no matter how valiantly each man strove, the same number would be destined not to marry.
Unlike other animal species, human societies can employ the power of law to constrain such positional arms races. In addition to whatever other purposes they may serve, laws against plural marriage may function as positional arms control agreements that make life less stressful for men. And this may help explain their appeal to the predominantly male legislatures that enact them.
Robert H. Frank, an economist at the Johnson School of Management at Cornell University, is the co-author, with Ben S. Bernanke, of "Principles of Economics." E-mail: rhf3@cornell.edu
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