80% of all HIV positive women in the world live in sub-saharan Africa. This is the only region in the world where more women than men are living with HIV — scholars have referred to this phenomenon as the “feminization of HIV.”

How did this happen? A new study offers compelling evidence that one specific legacy of an African country’s colonial past can contribute to the high rates of women living with HIV decades later.

The paper, by Siwan Anderson of the Vancouver School of Economics, finds that the legacy of the legal system of a country’s former colonizer has a significant effect on HIV rates in the country. The peer reviewed paper, which is pending publication in the American Economic Review, finds that countries that use the common law system have significantly higher female HIV rates compared to countries that adopted the civil law traditions.  This is true even though the rates of male HIV are statistically insignificant between common and civil law countries.

The difference is remarkable.

Common Law vs Civil Law

Common Law is the legal tradition of the United Kingdom and most of its former colonies (including the United States). Civil Law is the legal tradition of continental Europe, including France, Belgium, Portugal, Spain and Italy which each had colonies in sub-saharan Africa. The legal traditions mostly developed separately from each other for centuries. This includes how these different legal systems approached property rights for women in general and married women in particular.

In the common law tradition married women did not have any property rights independent from their husbands. That changed in the late 1800s, when the U.K adopted the “Married Women’s Property Act” which allowed married women to own their own separate property in some circumstances. But if the marriage ended by death or divorce, the woman did not have any right to any common property. The civil tradition, by contrast, presumed that married couples owned property jointly, and upon the dissolution of the marriage the woman would be entitled to an equal share.

In the 1960s, legal codes in western countries became far more egalitarian, thus practically erasing any real difference in how continental Europe and western common law countries treated women’s property rights. But the 1960s was also a period of rapid decolonization, and newly independent countries in sub-saharan Africa did not update their legal codes accordingly.

And here, Anderson demonstrates how the legacy of these divergent legal traditions has a significant impact on female HIV rates.

It all comes down to “female bargaining power.”

Women in common law countries are less able to negotiate sex with their partners. Even if they suspect their partner may be HIV positive, they are less able to refuse sex. It also means that if they use contraception at all, they are more likely to use contraception that does not require anything of their partners–like IUDs, injections or the pill. (Incidentally, these forms of contraception do not much reduce their risk of contracting HIV.)

By contrast, Anderson finds “women in civil law countries are more likely to use contraception techniques that reduce their chances of contracting HIV, but also require compliance from their partner, such as condoms, abstinence, and the withdrawal method.”

Anderson calls this “female bargaining power.” In other words, being able to refuse sex and threaten divorce is, in general, far less economically devastating in civil law countries than in common law ones. The access to property rights empowers women to negotiate healthier outcomes with their husbands.

There is a large volume of academic literature that measures women’s social and economic empowerment against various health outcomes. But this is the first paper to correlate the common law legal system and it’s historically weak property protections for married women to HIV prevalence. The upshot, as Anderson writes, is that “large-scale legal reform, towards improved marital property rights for women, could potentially alleviate some of this extreme gender disparity in disease burden.”

This is yet more evidence that more legal protections afforded to women result in improved health outcomes.