[Paleopsych] Marginal Revolution: Sex on the Margin

Premise Checker checker at panix.com
Wed Nov 16 22:39:50 UTC 2005

Sex on the Margin

Sexual preferences are primarily biological in origin.  But sexual choice is 
about preferences and constraints.  Raise the price of sex with women and more 
men will choose to have sex with other men - that's what happens in prisons. In 
a [97]remarkable paper, Andrew Francis (a graduate student at the University of 
Chicago) examines how AIDS has changed sexual choice.  With admirable 
precision, Francis lays out the price of sex:

   ...it is thousands of times more likely that a male would get HIV having sex 
with a man than having sex with a woman.
   In terms of AIDS-related mortality, the expected cost of having unprotected 
sex once with a man is almost $2000, while
   the expected cost of having unprotected sex once with a woman is less than a 

Thus AIDS changes the price of sex, do we observe changes in choice? Francis 
wants to be careful about causality so he uses a clever instrumental variables 
approach.  He reasons that knowledge of AIDS and thus responsiveness to price 
is correlated with knowing someone who has AIDS and that knowing someone who 
has AIDS is exogeneous to other factors influencing sexuality.  Unfortunately, 
it appears that he only has information on whether a relative has AIDS and 
genetic factors mean exogeneity is unlikely to hold.  In fact, we would 
probably expect that simply knowing someone with AIDS is positively correlated 
with being homosexual (especially in 1992 when the survey was taken).

Indeed, Francis finds, as expected, that women who have a relative with AIDS 
are more likely to be engage in homosexual acts and identify as being 
homosexual.  But Francis finds that men who have a relative with AIDS are 
significantly less likely to:

   ...have had sex with a man during the last sexual event...have had a male 
sexual partner in the last year... say they
   are sexually attracted to men...rate having sex with someone of the same 
gender as appealing...[or] think of themselves
   as homosexual or bisexual.

The tendency to greater homosexuality among women and less among men is exactly 
what the economic theory predicts given how AIDS affects the price of sex. 
Genetic and social factors will have greater difficulty resolving this 
bifurcation so I think Francis has the upper-hand on the argument, although 
there may be counter-arguments based on the [98]gay-uncle theory).

Importantly, note also that Francis finds that not only is sexual choice 
malleable, as the prison story I opened with suggests, but so are sexual desire 
and identity.  At least on the margin!  (A point that non-economists are likely 
to miss.)

Thanks to Emily Oster for the pointer.

   97. http://home.uchicago.edu/~afrancis/research/Economics_of_Sexuality.pdf

More information about the paleopsych mailing list