NAIROBI - Circumcision usually depends on religion, tribe or culture. Now it seems likely that the loss of a small flap of skin can help prevent the loss of life from AIDS. But this conclusion is far less simple than it might seem. The relationship between circumcision and HIV has been hotly debated for over a decade. Many epidemiologists have argued that, if there is a relationship, it is not a matter of the removal of the foreskin but of the sexual behaviour of the cultural, ethnic or religious groups that are either circumcised or not circumcised.

Research by a Kenyan microbiologist, Maina Kahindo, challenges this theory. This was part of a larger study designed to investigate whether levels of risky sexual behaviour really were the deciding factor. Places in Kenya and Zambia, where more than a quarter of the people being studied were infected with HIV, were compared with places in Benin and Cameroon, where HIV prevalence was below 6%. To the surprise of the researchers, the only important difference in sexual behaviour was that risky sex was most common in low-prevalence Cameroon. The real difference lay in male circumcision: over 98% of the men in the West African areas were circumcised, compared with about 30% in Kenya and 10% in Zambia.

The most compelling evidence came from the Kenyan study, where circumcised men could be compared with men of the same Luo group and Christian religion who were not circumcised but who had similar numbers of sexual partners and rates of condom use. The results: 26% of uncircumcised Luo men were HIV-infected, compared with 6% of circumcised Luo men. The difference persisted even after controlling for other sexually transmitted diseases which increase HIV transmission.

The result is entirely plausible. Put simply, the tip of a circumcised penis is tougher than the exposed tip of an erect penis usually covered by a foreskin. This toughness protects against micro-lesions which give HIV an entry point to the body.

But it is not easy to know what to do with this information. Although the results have been presented at medical conferences, including an international conference on AIDS and other sexually transmitted diseases in Zambia in September, the controversial nature of the findings has researchers dancing in circles. In Africa, circumcision is a badge of tribal or religious affiliation. Along with death rites, it is probably the most deeply held of cultural traditions. In the fractious societies of East and Southern Africa, a government dominated by one ethnic group would be ill advised to override the cultural norms of another, even in the interests of reducing the spread of a disease that already causes four out of five young adult deaths.

Governments are unlikely to launch campaigns promoting universal circumcision. People will probably be left to decide for themselves what to make of the information. Public-health officials are deeply worried that circumcised men will decide they need do nothing more to protect themselves. Already, dangerous rumours are circulating that circumcision is a “natural condom”. And anything that undermines the campaigns to promote safer sex would be calamitous.