Monday, 31 March 2014

Is sex like driving? HIV prevention and risk compensation

The title of this post is the title of a paper last month in the Journal of Development Economics (ungated version here), by Nicholas Wilson of Reed College, Wentao Xiong of Harvard, and Christine Mattson of the University of Illinois, Chicago. I posted a couple of weeks ago on offsetting behaviour, so I thought I might address this paper as well.

There has long been some concern over the extent of offsetting behaviour that may result from new health interventions that reduce the risk of HIV transmission. In public health circles, this offsetting behaviour is known as "risk compensation", and basically it works the same way as I described earlier. If a new health intervention or health technology (e.g. adult male circumcision as in the Wilson et al. paper, or a hypothetical HIV vaccine as in this paper by myself and others (ungated earlier version here)) reduces the risk of acquiring HIV infection, then rational (or quasi-rational) people face lower costs of risky sexual behaviour. They will respond by engaging in more, or riskier, sexual activity (e.g. more unprotected sex). This reduces the impact of the health intervention, making it less effective overall (in terms of its reduction in HIV transmission in the population).

The authors investigate the impact of adult male circumcision on subsequent risky sexual behaviour using a randomised controlled trial (RCT) among men in Kisumu, Kenya (in some future post, I aim to talk about the wonderful world of RCTs in economics, but for now if you are interested I suggest you read this article from The Economist last year). One interesting and important aspect of this study is that they investigated the effect of risk compensation depending on whether the respondent believed that male circumcision was effective in reducing their risk of HIV infection (since those who don't believe it is effective won't engage in risk compensation). In other words, they compare the effect of circumcision on the 'believers' with the effect on the 'non-believers' (as well as a control group of uncircumcised men). I initially thought this was a good innovation, but on reflection I' not so sure (more on that later in the post).

Surprisingly (or unsurprisingly, depending on what literature you have already read on the topic, if any), the authors find no evidence of risk compensation. In fact, they find quite the opposite effect:
The results of our empirical analysis suggest that the behavioral response to circumcision among believers net of the response among non-believers was a reduction in risky sexual activity. That is, we find what appears to be a behavioral response that is the opposite of the risk compensation hypothesis.
How to explain this result? The authors suggest that the marginal cost of risky behaviour actually increases, unlike the traditional Peltzman effect where marginal cost decreases, and this leads to less risky behaviour. Why would marginal cost increase rather than decrease? There are two components of marginal cost of risky sexual behaviour. First, there is the chance of getting infected by HIV - based on various studies, this decreases when a man is circumcised (and remember that in this study they are comparing the men that believe this with those that don't). Second, there is the cost of dying young (foregone income, time with family, leisure, and all the good things of life, etc.). The authors argue that, because circumcision reduces the lifetime chance of acquiring HIV, it substantially increases life expectancy. So, while the first effect (lower chance of infection) reduces the marginal cost of risky behaviour, the second effect (higher life expectancy) increases the marginal cost of risky behaviour and more than offsets the first effect. They also have some evidence to support this, which arises because of the way they organised their research by beliefs:
...our results indicate the existence of a behavioral response that was not due to a perceived reduction in the HIV transmission probability. Namely, circumcised males who did not believe that circumcision is effective at reducing HIV transmission appeared to increase their risky behavior.
Of course, if you believe that people are rational, there is an alternative explanation for these results. If people in the trial have complete (or near-complete) knowledge (as we might assume that rational decision-makers are), then they will know beforehand that when their risk of HIV infection decreases, they will respond with riskier behaviour (we could call them 'rational risk compensators'). That is, they foresee their own risk compensation. So, when these 'rational risk compensators' are asked: "Do you believe that male circumcision increases, decreases, or does not influence your risk of acquiring HIV?" (the actual question used in the Wilson et al. study) they may answer "increases" or "does not influence". This would erroneously categorise the 'rational risk compensators' into the non-believers category (when in fact they are believers, but also believers in risk compensation). Then, when we look at the data on the non-believers category (which includes the 'rational risk compensators'), we might observe an increase in risky behaviour within that group following circumcision (provided the number of 'rational risk compensators' is large, relative the the number of true non-believers). This story is also consistent with the data in Wilson et al., including their results that suggest that circumcision (independent of the effects on beliefs) is associated with an increase in risky behaviour.

Indeed, when the authors ignore the difference between believers and non-believers, there is some evidence of risk compensation, with men less likely to say they always use a condom or that they used a condom the last time they had sex (both indicators of lower risk behaviour). To eliminate this alternative explanation for the results, it would have been interesting to see whether the research participants believed beforehand that they would engage in risk compensation.

So, given this alternative explanation, maybe the demise of risk compensation has been greatly exaggerated.

Addendum: Risk compensation in children's playgrounds (Conclusion: If you make the playground safer, children respond by playing harder leading to more long-bone injuries like broken arms or legs).

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