IN AIDS epidemiology, one orthodoxy—particularly in Africa, where things are at their worst—is that the main route of transmission is male promiscuity. Of course, in this most political of diseases, political correctness is de rigueur, so it is not put quite like that. But the assumption is that men give it to women. Men give it to other men. And to the extent that women give it to men, it is because they have acquired it in the first place in unequal partnerships, such as prostitution, based on male power or male patronage.

There is a lot of truth to this, of course. But perhaps not quite as much as orthodoxy would have it. For work by Vinod Mishra of Macro International, a research firm under contract to the American government, suggests women are not always the innocent vessels that HIV epidemiology takes them for. And that, in turn, means the models that epidemiology relies on may be wrong.

Dr Mishra presented his findings at the HIV/AIDS Implementers' Meeting held last week in Kigali. He had examined medical and demographic surveys from 11 African countries in order to extract data on what are known as discordant couples. In the field of AIDS research, a couple is deemed discordant when one partner is infected with HIV whereas the other is not.

The orthodox view would have predicted that when a couple is discordant, it is more often the man than the woman who is infected. In seven countries this did indeed turn out to be so. In the other four, however, it did not. Côte d'Ivoire and Kenya led, with 62% of discordant couples being female-positive in both countries. And even in Lesotho, where the female-positive discordancy rate was lowest, it was still 34%.

That is a surprise, but might be explained by women having been infected before they settled down. However, in a significant number of cases of female-positive discordancy, that settling down had taken place ten or more years in the past. In these cases it is hard to conceive of any explanation other than infidelity on the part of the woman—a hypothesis supported by the observation that female-positive discordancy is more common in polygamous marriages, where a wife is competing for attention from her husband, than in monogamous ones.

The reason this matters is that, hitherto, policies designed to curb the spread of AIDS by curbing infidelity have relied on surveys that asked people about their behaviour. In these surveys, married and cohabiting women (Dr Mishra did not distinguish between the two) generally claim to be faithful, whereas many men admit to adultery. Campaigns intended to change behaviour have thus been aimed mainly at the men.

That now looks naive. It probably always was. People are notoriously untruthful about their sex lives. Men, for instance, routinely claim in surveys conducted all over the world to have slept with more women than vice versa—an observation that a moment's thought shows is impossible. That should probably have sparked scepticism in the past. Nevertheless, for AIDS policymakers, a little more heterodoxy, and a little more realism, may now be on the cards.