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Surprising Science

HIV Prevalence in Africa Explained by “Marital Shopping”

If the transmission rate of HIV is low, then how have so many young women on the continent become infected?

There are two facts about HIV that are difficult to reconcile. The first fact is that the transmission rate of the disease is extremely low; the risk of being infected from an person who has the disease through vaginal intercourse is about one in a thousand or 8-12% per-partner-year.* The second fact is that the disease has an extremely high prevalence among heterosexual women in Sub-Saharan Africa; 40% of pregnant women in Botswana and 25% in South Africa are infected with the disease.

The question is then, if the transmission rate is low then how did so many young women become infected?

It may be tempting for those of us in the West to believe that the prevalence of HIV in Africa is related to high rates of promiscuity and commercial sex. But Jeremy Magruder, a young economist at the University of California at Berkeley, is about to publish an extremely compelling argument that shows that African women have become infected with HIV through doing what we all do; shopping for a mate.

In brief, his argument goes like this. Women and men in Africa experience a brief period in their lives while searching for the right marriage partner in which they are in a series of monogamous relationships with high turnover. This searching behavior generates a constant pool of individuals in short-term relationships. Rates of HIV transmission may be low on average, but the probability of transmission from a person who has become recently infected is as much as 10 times higher. So, introduce one recently infected person into this pool of people who are searching for a mate and the whole pool are at risk of being infected.

The paper finds that the introduction of one person per hundred into a searching pool will lead to an HIV prevalence similar to that in Kenya or Tanzania. The introduction of just three infected people per hundred into the searching pool will lead to South Africa’s epidemic prevalence rates.

Only a small fraction of sexually active people need to be engaging in risky behaviors in order to create the pandemic despite low transmission rates.

So why are so many women infected with HIV in Sub-Saharan Africa compared to the West, where women engage in the same type of searching behavior?  The difference is that in Western countries a sufficient number of people in the marital searching pool use condoms in short-term relationships.

In my mind, the most important implication of this research is that reducing HIV prevalence in Africa to North American levels does not require a major change in African social norms nor would it require a major medical intervention. It doesn’t even require that 100% of single people searching for a mate use condoms during sex. The economic model suggests that if only 50% of the participants in the marital searching pool use condoms for the first three months of their relationships, the prevalence rates would drop to those comparable to the West.

Health campaigns that strive to convince people that they should use condoms for the rest of their lives, even after marriage, have not been effective. Everyone wants to have sex without a condom eventually. If you know that you won’t be using a condom in six months, and believe that the risk of infection then is the same as it is now, then why use one today?

A public health campaign targeted to young single people encouraging condom use in the first three months of new sexual relationship is bound to have a bigger effect on behavior. If it isn’t promiscuity that is killing women in Africa, but marriage, then this small change in behavior could be what is needed to save the lives of millions of women and children.

* This transmission rate of 8-12% is the African per-person year (PPY) transmission rate. The PPY rate in the US and European is in the 5-10% range.


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