A national survey in Australia has found that half the teenage girls and 30 per cent of boys who are sexually active do not use condoms.
Among 16-year-olds, only about 40 per cent said they were, or would be, faithful to their partner.
According to Professor Susan Moore, a psychologist at Victoria University in Melbourne, young Australians use various types of "distorted reasoning" to justify unsafe sexual behaviour.
The persistent beliefs which encouraged unsafe behaviour included such notions as "the male sex drive must not be constrained", "it's what adults do" and "there is no such thing as safe sex, so why bother?"
Young Australians were well informed about sexually-transmitted diseases and condoms, yet public education campaigns often ignored the situations in which adolescents made decisions about sex.
"While persisting with condom use is sometimes interpreted as undermining the assumption of love and trust among young people, this belief has not been adequately addressed by public education," Professor Moore said.
She said that highly moralistic messages urging young people to "just say no" were ineffective. Such messages might be effective in changing intentions about sex but they were "drowned out" in situations of high arousal.
"Young people feel the excitement of sex, and it is emphasised in media portrayals. If safe sex is portrayed as boring and conservative, its uptake will be particularly limited among young people still experimenting with sexual possibilities," said Professor Moore.
Meanwhile, another Melbourne researcher has adopted a new approach to teaching teenagers about sex, using the fact that most learn about it from their peers, not from their parents or teachers.
Professor Roger Short, from the Royal Women's Hospital, developed a sex-education programme which is run by third-year medical students.
"The medical students are young enough to relate to teenagers, but they've also learned enough to know what they're talking about," Professor Short said.
The students participate in sex-education programmes at secondary schools and run a "Safe Sex Tent" during university freshers' week.