LONDON, March 26 2004 (Reuters) - Circumcised men are less likely to be infected with the virus that causes AIDS because of biological reasons and not less risky behaviour, scientists said on Friday.Studies have shown that men whose foreskin has been removed are six to eight times less likely to become HIV positive but there has been some debate about the reason for the lower infection rate. Researchers at Johns Hopkins University Medical School in Baltimore, Maryland found that circumcision had a protective effect against HIV, but not against other sexually transmitted infections (STIs) such as syphilis or gonorrhoea. "The specificity of this relation suggests a biological rather than behavioural explanation for the protective effect of male circumcision against HIV," Dr Robert Bollinger said in a report in The Lancet medical journal. Although male circumcision is common in the United States, the practice varies throughout the world and is influenced by cultural and religious attitudes. Bollinger and his team studied men in India, where circumcision is not common, between 1993 and 2000. All of the 2,298 men were attending one of three sexually transmitted disease clinics and were HIV negative at the start of the study. Their HIV status and risk behaviour were assessed regularly. "These data confirm previous findings that male circumcision reduces the risk of HIV acquisition," said Bollinger. Because circumcision did not prevent the men from infection with other STIs, Bollinger believes the study supports the hypothesis that protection is due to the removal of the foreskin, which contains cells that have HIV receptors which scientists suspect are the primary entry point for the virus into the penis. "Our results suggest that the foreskin has an important role in the biology of sexual transmission of HIV," he said. Some researchers have recommended male circumcision as a means to prevent the spread of HIV. Bollinger and his team called for clinical trials, where culturally acceptable, to assess the safety and effectiveness of male circumcision as a toll against AIDS. They also stressed the need for new compounds to block the entry of the virus into the cell.