Preventing the sexual transmission of HIV is an urgent public health priority. With more than two million people newly infected with HIV every year, lives hang in the balance.

At the same time, messaging about preventing the sexual transmission of HIV is the ultimate act of cross-cultural communication. In our haste to save lives, it can be easy to make blunders in that kind of communication.

Take for instance historical assumptions about drivers of the HIV epidemic in sub-Saharan Africa. It was long assumed that a major driver of the vastly greater prevalence of HIV infection in Zimbabwe, South Africa and other epicenters of the African HIV epidemic is a high number of concurrent sexual partners. Yet, a closer examination of the evidence has not borne this out.

Recently, a similar cross-cultural assumption about African sexual practices has been called into question.

That assumption has to do with intergenerational sex. Much ink has been spilled on how intergenerational sex helps drive HIV transmission in the developing world. Publicly funded prevention campaigns aimed at reducing intergenerational sex have proliferated, including from reputable outfits like USAID. For example, the “Something for Something Love” campaign in Uganda seeks to inform youth and others about the risks of transactional sex between young women and older men:

Likewise, in the South African province of Kwazulu-Natal, the public health department launched a “Say No to Sugar Daddies” campaign that placed 89 billboards across the province warning against the dangers of cross-generational sex:

Intergenerational HIV transmission scenarios are absolutely plausible. The typical story is that an older, wealthy, HIV-infected man coaxes a younger, poor woman into a semi-consensual relationship in which her self-protective instincts are subverted by economic need. Many people – myself included – have assumed intergenerational sex is a driver of HIV transmission.

Yet is intergenerational sex really a driver of the African HIV epidemic?

Early studies suggested it might be. Surveys showed that that intergenerational sex occurs in Africa – as everywhere – and that many Africans believe such relationships enhance the risk of HIV transmission. Intergenerational sex was also associated with lower condom use, perhaps in part because it often occurs during marriage in which condom usage is commonly lower than among casual contacts. Other researchers found that intergenerational sex had clear economic drivers, with women using such relationships for economic gain and social status.

Yet even these early studies of intergenerational sex in Africa suggested the picture was more complicated than originally suspected. Some women took pleasure in intergenerational sexual relationships and claimed a higher degree of autonomy than predicted. Some involved in intergenerational sex made intriguing social assumptions about the activity. For instance, some frowned upon women not in intergenerational sexual relationships as not taking full advantage of potential partners.

Ultimately, the real question is whether intergenerational sex drives the burgeoning HIV epidemic in Africa and other developing world countries?

In answer, researchers writing in Journal of Acquired Immune Deficiency Syndromes in August assessed age gaps between 2,444 women aged 15-29 and their sexual partners in the South African province of KwaZulu-Natal, and then formally assessed if women engaged in intergenerational sex were more likely to develop new HIV infection as detected by annual testing. In this high risk population, fully eight out of a 100 women contracted HIV infection during follow up, giving the researchers ample opportunity to assess whether intergenerational sex (and other factors) were associated with enhanced risk of HIV infection.

Contrary to expectations, the high-quality, longitudinal study showed that participation in intergenerational sex did not impact the likelihood of contracting HIV infection. This was true in simple univariate analyses, and in several multivariate analyses including some that adjusted for sociodemographic characteristics such as age, income, marital status and educational level. From every angle, intergenerational sex and the risk of HIV infection were simply not associated.

The authors’ hypotheses for why there was no association between intergenerational sex and HIV transmission risk illustrate why identifying social drivers of HIV transmission is notoriously challenging. They point out that while older men were more likely to be HIV-infected, they were also more likely to be chronically infected and on antiretroviral therapy. This means older partners, though more likely to have HIV infection, would have had less transmissible HIV. By contrast, when younger men were infected they were more likely to have recently seroconverted (and thus be more infectious) and also less likely to be on treatment. In addition, women whose partners were closest in age to them had a higher number of casual partners in the past year. This suggests that both men and women in intergenerational partnerships behave differently from those not in intergenerational relationships, and these differences may both protect from HIV transmission and increase HIV transmission risk in ways that cancel each other out.

This is not to suggest that intergenerational sex is an entirely benign thing, nor to intimate that intergenerational sex has been definitively exonerated as a driver of the HIV epidemic. Intergenerational sex can be transactional sex, and thus associated with decreased autonomy and reduced HIV prevention activities, and so confirmatory studies will be important to conduct in different settings where the pros and cons may line up differently.

In the meantime, the most critical implication of this study is that researchers and public health authorities should be cautious about making assumptions about sexual behavior in Africa, particularly when those assumptions travel across national or continental boundaries.