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 people under 25 years of age 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.

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

"This study summarizes key qualitative findings that help in understanding young people's sexual behavior 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 honor killings, in which a woman who is thought to have dishonored 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 programs have not been effective," the researchers said.

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