Parents can use a variety of measures to filter unwanted material

This included talk about sex and invitations to sexual activity. One child even reported being sent a ticket to travel to meet an online correspondent.

The older teenagers, especially girls, were the ones who reported most approaches.

Online stress

But, perhaps not surprisingly, it was the younger children who were most distressed by them.

The parents of the Internet users applied a variety of measures to supervise them, including having rules about what they could do online, inspecting files and discs, and using software that blocks certain types of websites.

But these appeared to make no difference to the rate of sexual approaches.

It is not clear whether the findings might be replicated among Internet users outside the United States, but of course most websites are accessible to any user, regardless of which country he or she is in.

The researchers, writing in the Journal of the American Medical Association, say parents, teachers and health professionals should be ready to deal with the distress caused by online sexual approaches.

But they stop short of advising parents to ban their children from using the net.