Social influences often prevent young people from using condoms, say researchers who reviewed more than 250 studies on the sexual behaviour of under-25s.

The study concluded that social expectations of how men and women should behave hamper safer sex campaigns in many countries.

"Our findings help to explain why many HIV programs have not been effective," said Dr. Cicely Marston of the London School of Hygiene and Tropical Medicine, who led the review.

"Giving out condoms and information is vital, but it is not enough. Even where young people know about the importance of condoms, social factors — in particular stereotypes about how men and women should behave and a reluctance to talk openly about sex — hamper their use.

Safer sex campaigns need to tackle these issues if they are to succeed."

The sexual freedom of women is restricted worldwide compared with men, although the penalties vary widely.

"Worldwide, not only is sexual behaviour strongly shaped by social forces, but those forces are surprisingly similar in different settings, with variations of the extent to which each theme is present rather than of kinds of themes," the researchers said in Saturday's issue of the medical journal The Lancet.

Under gender stereotypes for example, young men often feel pressured into having sex when given the opportunity, regardless of whether they have a condom.

In contrast, young women often feel their reputation will be tarnished if they carry condoms, although they are expected to be responsible for preventing pregnancy.

Young people the world over said they found it hard to talk about the possibility of sex with potential partners, making it difficult to plan condom use.

The under-25s tended to try to guess the HIV status of potential partners using unreliable indicators such as how "clean" someone looked or how well they knew them socially.

Condoms were also associated with a lack of trust, the researchers found.

The studies were carried out between 1990 and 2004.