|Taking one for the team?|
This is probably familiar to lots of you, but the nature of lots of preventable illnesses in poor countries is such that risk externalities can be pretty subtle and the solutions much less obvious. To take one example, consider HIV transmission and concurrency vs. serial monogamy in sexual relationships. People in Southern Africa commonly have more than one concurrent sexual partnership; in contrast, people in Asia and the West have no fewer lifetime sex partners, but tend to have only one at a time. The practice of “concurrency” in sexual pairings has been shown to substantially increase transmission rates at a societal level, because the amount of HIV in the bloodstream is highest within the first month. If you don’t have sex with another partner until after that window passes, the odds that you’ll infect them drop substantially. Now suppose we convince someone to stop having more than one sex partner at a time, and instead become serially monogamous. Doing this might convince them to be safer in other ways as well: maybe they’ll have fewer sex partners or less risky sex, both of which would directly protect them. But if all they do is re-sequence their partners to have one at a time, then they get no direct benefit from serial monogamy at all. It is their partners that are protected. The risk benefit of serial monogamy per se, rather than other behavior changes, is entirely external. Even if people understand that serial monogamy is safer, we might see fewer people doing it than we’d like, as they cynically decide to protect only themselves.