SUNDAY marked the first day gay couples were allowed to marry in New York State. This provoked an unusual New York Times op-ed by lawyer Jonathan Turley. He reminds us of another group being robbed of their basic rights of citizenship—polygamists.

The reason might be strategic: some view the effort to decriminalize polygamy as a threat to the recognition of same-sex marriages or gay rights generally. After all, many who opposed the decriminalization of homosexual relations used polygamy as the culmination of a parade of horribles. In his dissent in Lawrence, Justice Antonin Scalia said the case would mean the legalization of “bigamy, same-sex marriage, adult incest, prostitution, masturbation, adultery, fornication, bestiality and obscenity.” Justice Scalia is right in one respect, though not intentionally. Homosexuals and polygamists do have a common interest: the right to be left alone as consenting adults. Otherwise he's dead wrong. There is no spectrum of private consensual relations — there is just a right of privacy that protects all people so long as they do not harm others. Others have opposed polygamy on the grounds that, while the Browns believe in the right of women to divorce or leave such unions, some polygamous families involve the abuse or domination of women. Of course, the government should prosecute abuse wherever it is found. But there is nothing uniquely abusive about consenting polygamous relationships. It is no more fair to prosecute the Browns because of abuse in other polygamous families than it would be to hold a conventional family liable for the hundreds of thousands of domestic violence cases each year in monogamous families.

Mr Turley claims he's not fighting for the state to recognise polygamous marriages, but he'd like to see the practice decriminalised. Though I am not quite sure what gay couples and polygamists have in common. The gay marriage cause is not about privacy. Rather, it's a quest to obtain equal rights, to ensure that gay spouses are protected, entitled to Social Security benefits, health insurance, and their partner's assets if the relationship ends through death or divorce. Extending these same rules to polygamy would be a fiscal nightmare. Could you imagine the expense of granting such privileges to someone with multiple spouses? Isn't Social Security already under-funded? Think of the cost to employers who must provide health insurance to one man and his seven wives. Imagine the litigation costs for a male breadwinner who dies unexpectedly without a will and with multiple dependent wives. Are assets divided evenly or based on how many children each wife produced? True, Mr Turely claims he's not asking for legal recognition of plural marriages, but his case does bring up some interesting economic questions.

Economists often argue that polygamy (we'll focus on men marrying multiple women because that tends to be more common than polyandry, but the logic still applies to a woman with multiple husbands) benefits women because it enhances their market power. That's because it means more marriageable men for every women. That may be true in the dating market, and as Gary Becker argues, probably explains why in polygamous cultures only wealthy men tend to have multiple wives. When a woman has more options, she will opt for a more productive and desirable spouse. Especially in an agrarian economy where a steady food supply is scarce, polygamy made sense, for men and women. Subsistence farming requires many children to provide free labour, so taking several wives was necessary. A woman would partake in the arrangement, because being part of a larger farm provided more safety, security, and resources for her and her children than being the only wife of a poor man.

But once a woman enters into a polygamous arrangement, it seems she'd have less power. Bargaining power in a household is often based on who contributes what to household production and utility. Each person provides certain services and resources to make the household function and this keeps the marriage balanced. But the power structure is different when you have one man and several women. The marginal value each woman can uniquely provide diminishes the more women that are added to the family. In contrast, a male's services become relatively scarcer, so he will have more power within the marriage. That is unless the wives manage to collude, though cartels tend to be inherently unstable.

You might argue that a woman retains some power because she can leave marriage and find someone else. After all, she has exceptional market power in the dating market. But terminating a marriage is costly, especially if the woman has children and is dependent on the man financially (and has no legal recognition). Also, in the sort of misogynistic cultures where polygamy thrives, a large premium is placed on a women's ability to bear children. Under these circumstances her market value declines as she ages.

For fiscal reasons alone, marriage should be limited to two consenting adults. But the question of whether polygamy should be a criminal activity is a difficult one. It may not make sense for a modern woman to enter into such an arrangement, but bad judgment is not illegal. Also, lots of marriages have odd power dynamics; the state does not charge these couples with criminal activity. But polygamy may pose some negative externalities. It often leads to a large pool of unmarried, young men which can lead to social instability. And polygamists tend to have many children to support which may prove to be a burden on the taxpayer. Rather than simply criminalising plural marriages, perhaps people should simply be taxed for each additional spouse.