Like the creators of Big Love, I’ve long seen strong parallels between gay marriage and polygamy.  After last weeks’ decision, some prominent pundits said the same.  But legendary Law and Economics scholar Richard Posner rejects the analogy:

[L]ater in his opinion the chief justice remembers polygamy and suggests
that if gay marriage is allowed, so must be polygamy. He ignores the
fact that polygamy imposes real costs, by reducing the number of
marriageable women. Suppose a society contains 100 men and 100 women,
but the five wealthiest men have a total of 50 wives. That leaves 95 men
to compete for only 50 marriageable women.

Do you know what else has exactly the same effect?  Female gay marriage!  If half of women lost interest in men, this would have the same “real costs” that Posner attributes to polygamy.  Indeed, the same goes for straight women who don’t date – or straight women with high standards.  Posner’s argument proves too much – especially considering the fact that female gay marriages outnumber male gay marriages. 

More critically, though, Posner’s argument ignores the fundamental economic distinction between transfers and deadweight costs.  Anything that raises the male/female ratio in the dating pool imposes costs on men.  Anything that lowers the male/female ratio imposes costs on women.  This is no different from shifts in supply or demand in any other market: While change implies both winners and losers, total social surplus is greatest at the intersection of supply and demand.

If Posner really wanted to economically critique polygamy, he would have focused not on distribution, but externalities.  In his earlier work, he did so – but he was still off his game. 

My view is that polygamy would impose substantial social costs in a
modern Western-type society that probably would not be offset by the
benefits to the parties to polygamous marriages. (For elaboration, see
my book Sex and Reason (1992), particularly Chapter 9.)
Especially given the large disparities in wealth in the United States,
legalizing polygamy would enable wealthy men to have multiple wives,
even harems, which would reduce the supply of women to men of lower
incomes and thus aggravate inequality.

Yes, but this extra inequality could have great incentive effects on male production.  Maybe more inequality – or a different kind of inequality – would be socially beneficial.  Posner continues:

The resulting shortage of women
would lead to queuing, and thus to a high age of marriage for men, which
in turn would increase the demand for prostitution.

So?  The standard evils of prostitution stem from prohibition, not prostitution itself.

Moreover, intense
competition for women would lower the age of marriage for women, which
would be likely to result in less investment by them in education
(because household production is a substitute for market production) and
therefore reduce women’s market output.

Even if you don’t think signaling is a big deal, the obvious response is again, “So what?”  Allowing women to inherit wealth also makes a life of uneducated leisure more attractive, but I doubt Posner wants to do anything about it.  And if intense competition discourages female education, wouldn’t it also encourage male education?

The only negative externality that Posner clearly identifies is political, but even he doesn’t take it seriously.

In societies in which polygamy is permitted without any
limitation on the number of wives, wealthy households become clans,
since all the children of a polygamous household are related through
having the same father, no matter how many different mothers they have.
These clans can become so powerful as to threaten the state’s monopoly
of political power; this is one of the historical reasons for the
abolition of polygamy, though it would be unlikely to pose a serious
danger to the stability of American government.

An unlikely danger indeed.  Imagine a polygamist reading Posner’s broad-brush history:

There is of course a long history of persecution of gay people, a
history punctuated by such names as Oscar Wilde, Pyotr Ilyich
Tchaikovsky, and Alan Turing. Until quite recently, many American gays
and lesbians took great pains to conceal their homosexuality in order to
avoid discrimination. They value marriage just as straight people do.
They want their adopted children to have the psychological and financial
advantages of legitimacy. They are hurt by the discrimination that the
dissenting justices condone. Prohibiting gay marriage is discrimination.

Change the labels and names, and the parallel between government persecution of gays and polygamists is nearly perfect.  Let the slippery slope of marriage equality proceed at maximum speed.