In light of yesterday’s decision by the Federal District Court in San Francisco to strike down a ban on same-sex marriage in California, why not ask the question: How does the legalization of gay marriage in a particular geographical region affect the HIV rate there?

The prevalence of HIV is a subtext of the same-sex marriage debate, whether or not we think it belongs there.  The speculation that legalizing gay marriage will lead to more HIV cases goes like this: Surely, the legalization of same-sex marriage is going to lead to increased tolerance of homosexuality. Increased tolerance might encourage some people to adopt a more active homosexual lifestyle. HIV rates are higher among homosexual men than among heterosexual men, so doing the mental math means surely HIV rates must increase.. Well, we don’t need to rely on speculation to see what the relationship between tolerance and HIV actually is—we have tools to measure this, and, as it turns out, tolerance is exactly what the doctor orders.

Tolerance towards homosexuality is easy to measure. For example, we can conduct a survey that asks if the following statement is true or false: “Homosexual behavior is always, or almost always, wrong.” We can then use the percentage of individuals who choose “true” as a measure of the tolerance of a society. By this measure, the United States, as a nation, is relatively intolerant towards homosexuality, with 60% of those surveyed responding “true”.* Compare this to a relatively tolerant country like Canada where the percent who said “true” is closer to 30%.

We can measure the prevalence of HIV as well and, as it turns out, the US has a relatively high HIV rate compared to other developed countries. In fact, the HIV rate in the US is double that in any other OECD country.**

This doesn’t prove a relationship though between tolerance and HIV, because we haven’t controlled for a whole variety of factors that might affect both tolerance and HIV. Perhaps, for example, people become intolerant when HIV rates are high. Perhaps increased public education increases tolerance and decreases HIV, or maybe changes in behavior both decrease HIV and increase tolerance. Fortunately, we have methods to weigh these issues and keen researchers who can provide empirical evidence on the relation between tolerance and HIV.

New research, which looks at variations between US states in tolerance towards homosexuality and HIV rates, has found that an increase in tolerance significantly reduces the rate of HIV.***  Importantly, given the current debate, they also find that laws that ban marriage between individuals of the same sex increase HIV rate by between 3 and 5 per hundred thousand. If we extended this analysis to California, the marriage ban in that state could have contributed to between 1,092 and 1,820 cases of HIV.

This result is not as counter-intuitive as you might think. Increased tolerance reduces risky sexual behavior on both the intensive and extensive margin. On the intensive margin, for men who already have an active homosexual lifestyle, increased tolerance might make it possible for them to continue that lifestyle in way that encourages less risky sex. For example, increased tolerance might reduce the need to meet in parks and restrooms for anonymous sex by making other meeting places, like clubs, less dangerous for them. This is consistent with evidence that US states that are more tolerant have less high-risk meeting places and more  safe meeting places.

On the extensive margin, men who do not have an active homosexual lifestyle because they either fear the repercussions, or strongly desire to marry and have a family, might now choose an active homosexual lifestyle if there is an increase in tolerance or if marriage bans are removed. If these men, on average, behave in a less risky way, we would expect an over-all decrease in risky sexual behavior among homosexual men because more risk-adverse individuals are now part of the pool.

All this reminds me of that old saying; if you are not part of the solution you are part of the problem.

This post originally appeared on my original blog Dollars and Sex at Big Think.

*2006 General Social Survey


***Tolerance and HIV by Francis, Andrew M.; Mialon, Hugo M.; Journal of Health Economics, March 2010, v. 29, iss. 2, pp. 250-67.