Auto Accidents, AIDS, Contraception and the Pope

Tuesday, April 28th, 2009

David Friedman draws some unusual connections between auto accidents, AIDS, contraception and the Pope:

Suppose you make cars safer by requiring seat belts, collapsible stearing columns, and other changes that make it less likely that an auto accident will kill the car’s occupants. The obvious conclusion, and the one many people reach, is that the highway death rate will go down.

Sam Peltzman, in a classic article, pointed out that there was no good theoretical reason to expect that to happen. Auto accidents do not simply happen; they are the result of decisions made by drivers, such as how fast to drive, how much attention to pay to driving and how much to conversations with your passengers or listening to the radio, whether to drive home or take a cab after drinking a little too much. Making cars safer lowers the cost of dangerous driving; on the margin, drivers are more willing to risk accidents the less likely accidents are to kill them. So making cars safer results in fewer deaths per accident but more accidents. There is no theoretical basis to predict whether the net effect will be fewer deaths or more. Peltzman offered statistical evidence that, in the particular case he he was looking at — a collection of safety requirements imposed in the 1960′s — the two effects roughly cancelled. Death rates per accident went down, the accident rate went up, and the annual death rate was about what it would have been without the changes.

Perhaps you see the parallel with AIDS and contraception:

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.”

Friedman suspects that neither side of the controversy is being entirely honest about its objectives:

Unreliable forms of contraception [like the Church-approved rhythm method] can work pretty well for holding down marital birth rates. On the other hand, if your objective is to permit women to have sex with men they aren’t married to without a significant risk of pregnancy — to permit, in other words, what has become the normal pattern of sexual behavior in developed societies — there is much to be said for more reliable forms of contraception [like condoms].

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