Wednesday 15 April 2009

Offsetting Behaviour: condom use edition

About a month ago, the Pope released a statement arguing against reliance on condoms in the fight against AIDS in Africa. He was excoriated for this view. David Friedman today wonders whether there might not be offsetting behaviour in condom use in Africa.
Just as with auto safety and auto accidents, making sex safer has two effects working in opposite directions. It makes the chance that a given act of sex will result in AIDS transmission lower. But, by lowering that risk, it reduces the incentive to avoid sex entirely, to avoid sexual acts such as anal intercourse that are particularly likely to transmit AIDS, to avoid sex with people likely to give you AIDS, such as prostitutes. On theoretical grounds we have no way of knowing whether the net effect will be more AIDS or less.

It turns out that there is evidence that, just as in the auto case, the two effects roughly cancel. That, at least, was the widely reported conclusion of a Harvard AIDS researcher who had actually looked at the data. “We have found no consistent associations between condom use and lower HIV-infection rates, which, 25 years into the pandemic, we should be seeing if this intervention was working.”
Todd Zywicki over at Volokh previously pointed to stronger confirmation of this offsetting effect. The Alligator, linked to by Zywicki, quotes Dr. Edward Green, Director of Harvard's AIDS Prevention Research project:
In epidemics that are population wide, where most HIV is found in the general population, for whatever reason we can't get people to use condoms consistently, and when they use them at all, that seems to have the effect of disinhibiting people's behaviours so they end up taking greater sexual risks and cancelling whatever risk reduction they have gotten from the technology they're using.

Condoms make you feel safer, so you take more risks. Always beware the offsetting behaviour.


  1. Yes, there is offsetting behavior with respect to aids. However, a world with an equal number aids cases yet *more* sex looks like a net improvement. (Surely the individuals see it this way, or they would not change their behavior when the price of sex falls.) Driving with seatbelts has benefits too: one can reallocate some attention to the radio or a conversation for approximately the same health risk as no seat belt. Potential harm to pedestrians leaves the case ambiguous here, but I do not see a similar externality with respect to sex. (David Friedman points to an argument that contraception leads to more single mothers, but in the context he sets up, this is not clearly a negative.)

  2. Oh, I'd never deny that folks are optimizing or that they derive utility from being able to shirk on the safety safeguards. Just like I'm on a higher indifference curve if I get to play more with the radio for a given level of safety if my car is safer, I'm also on a higher indifference curve if I get to have more fun for a given level of risk if I'm using condoms.

    It's more from a policy perspective: if your goal is to reduce negative consequence X by means of subsidizing or mandating safety apparatus, a good chunk of your efforts will leak into consumption on other margins.