OK – it has been a worthwhile break. Let’s do this.
Ever since a spate of randomized controlled trials revealed that circumcision can functionally reduce a man’s chance of contracting HIV, global health officials have been been pushing mass circumcision campaigns as much as possible in sub-Saharan Africa. Concerned that the assumption that the internally-valid results from the RCTs may not hold in `the real world’, I wrote a fairly critical post a few years ago. My main concern was that no one was properly accounting for post-treatment behavioural changes.
I was particularly worried about something called the Peltzman effect, a concern that economists have about offsetting behaviour: if you reduce the riskiness of a particular activity for someone, then they have an incentive to do that activity more. While academics have had some trouble reliably identifying Peltzman effects in the real world, it is still a valid concern: while those undergoing an experimental treatment may not change their behaviour before more information on the effectiveness of that treatment is known, men who know that circumcision reduces their chance of contracting HIV might react by simply having more sex.
Someone has finally taken a look at this – Nicholas Wilson, Wentao Xiong and Christine Mattson have a paper specifically looking for the Peltzman effect in a group of Kenyan men who participated in one of these RCTs. They subdivide the sample based on their pre-treatment beliefs on the effectiveness of circumcision, then look at risky sexual behaviour in two follow-ups six and twelve months later. The punchline?
Contrary to the presumption of risk compensation, we find that the response due to the perceived reduction in HIV transmission appears to have been a reduction in risky sexual behavior. We suggest a mechanism for this finding: circumcision reduces fatalism about acquiring HIV and increases the salience of the trade-off between engaging in additional risky behavior and living longer. We also find what appears to be a competing effect that does not operate through the circumcision recipient’s belief about the reduction in the risk of acquiring HIV.
At first glance, this sounds like good news: those who said they believed that circumcision was effective reduced their risky behaviour – the exact opposite of what one would expect from a Peltzman-type relationship. Wilson et al. chalk this up to a sort-of income effect: now that their baseline risk of contracting HIV is lower (therefore raising potential life expectancy), subjects who have been circumcised decide to invest more in life quality, avoiding dangerous behaviours.
But wait – this nugget of good news is actually obscuring something else: those who didn’t believe that circumcision worked increasedÂ risky sexual behaviour – they were less likely to use a condom during intercourse and had more sexual partners. The authors don’t have any good explanations for this behaviour, other than suggesting that being circumcised might make men more attractive to other women, since it signals that they have a lower chance of having HIV. I’m not sure how plausible this is, unless men are making their circumcision status public, this is the sort of information that is usually only revealed very, very late in the game. How could we get around this? Randomly reveal men’s circumcision status (sounds fun, but unethical – these two sometimes go hand in hand)?
The increase in risk among the non-believers is disconcerting, even if in net terms there is no increase in risky behaviour. While the believers, who are in the majority, offset the non-believers, we don’t know what the aggregate effects on HIV outside of the sample would be. Nor do we actually see the effects of belief/non-belief on HIV status in this sample, which would be interesting and useful to know: Â we could then see to what extent the increase in risky practices offsets the large biological impact of circumcision.
While this is all very interesting, the results of the paper do rely on a strange parsing of the data: reported belief is useful, but might be picking up something else, such as general public health knowledge. Ideally, we’d have some sort of shock to beliefs, say from randomly treating someone to an intensive course on the benefits of circumcision, but perhaps this is asking too much.
What we should be concerned with is what the ratio of believers to non-believers looks like in other settings. The authors specifically recommend that circumcision programmes should come hand-in-hand with information campaigns. If reported belief is something that is malleable, then this is a reasonable strategy. If reported belief is actually a measure of something more inherent, then we’re in trouble.