LONDON - Social and cultural factors, not just unavailability or ignorance, influence why young people do not use condoms, researchers said on Friday.

Some sexually active under 25s associate condoms with a lack of trust, while others believe carrying them could imply sexual experience, which might be a plus for men but not necessarily for women.

Condoms are seen in a March 2, 2006 file photo. US efforts to promote abstinence as a cornerstone of sexual education have not lowered levels of sexually transmitted diseases, two former US surgeon generals said on Thursday. [Reuters]

In a review of more than 250 studies of young people's sexual behaviour, researchers at the London School of Hygiene and Tropical Medicine discovered striking similarities in what affects young sexual behaviour in different countries.

"This study summarises key qualitative findings that help in understanding young people's sexual behaviour and why they might have unsafe sex," said Dr Cicely Marston and Eleanor King in a study in The Lancet medical journal.

They found that compared to men, women's sexual freedom was universally restricted. Penalties for transgression varied from verbal criticism to honour killings, in which a woman who is thought to have dishonoured her family is killed.

The studies, carried out between 1990-2004, showed that young people assess a potential partner's disease risk, and the need for a condom, by their appearance and how well they know them socially.

Men were expected to be highly sexually active and women were expected to be chaste, according to data from countries including Britain, Australia, Mexico and South America.

"Our findings help explain why many HIV programmes have not been effective," the researchers said.

"Programmes that merely provide information and condoms, without addressing the crucial social factors identified are only tackling part of the problem," they added.