© AlamyThe study showed that young people who are exposed to sex on screen tend to lose their virginity at an earlier age and be more promiscuous
Not only are they more promiscuous, they are also more likely to engage in risky sex by not using condoms.
The six-year study of more than 1,200 teens refers to sexual content in films but campaigners against online porn say it could equally apply to videos on the internet.
They point out that children can now see a lot more sexual imagery online than they ever did at the cinema - meaning that the effect will be magnified. Pornographic images and videos are freely available on the net, many on sites with no age verification procedures at all, putting children at risk.
The NSPCC warns that young male teens are now pressurising their girlfriends to copy what they see on porn films downloaded from the internet.
Other reviews have found that exposure to porn makes boys more likely to view girls as sex objects.
The Daily Mail is campaigning for an automatic block on online porn. Over-18s would be able to see adult material only if they specifically opted in.
The study on the effect on younger teenagers of sex in films was published in Psychological Science, the peer-reviewed journal of the Association for Psychological Science.
Researchers from Dartmouth College, an Ivy League university in the US, surveyed 684 top-grossing movies from 1998 to 2004, and then coded them for sexual content.
A film such as Eyes Wide Shut would be coded highly, while Lord Of The Rings: The Return Of The King would get a low coding. Although some of the films had 18-ratings, they could have been seen by younger children on DVDs at home.
The researchers then recruited 1,228 youngsters aged between 12 and 14, and each was asked which films on the list they had seen from a number of different collections of 50 that were randomly selected.
Six years later the same participants were surveyed to find out how old they were when they became sexually active and how risky their sexual behaviour might have been.
The results indicated that exposure to sexual content in movies at an early age is likely to influence adolescents' sexual behaviour.
Lead researcher Dr Ross O'Hara said: 'Much research has shown that adolescents' sexual attitudes and behaviours are influenced by media. But the role of movies has been somewhat neglected, despite other findings that movies are more influential than TV or music.'
Speaking about the results of his study, Dr O'Hara said: 'Adolescents who are exposed to more sexual content in movies start having sex at younger ages, have more sexual partners, and are less likely to use condoms with casual sexual partners.'
Daily Mail campaign to block online porn
Between the ages of ten and 15, the tendency to seek more novel and intense stimulation of all kinds peaks. The wild hormonal surges of adolescence makes judicious thinking a bit more difficult.
Dr O'Hara said that greater exposure to sexual content in movies at a young age actually led to a higher peak in 'sensation-seeking' during adolescence.
He said: 'These movies appear to fundamentally influence their personality through changes in sensation-seeking, which has far-reaching implications for all of their risk-taking behaviours.'
And although Dr O'Hara said it was impossible to prove a direct causal effect between such films and sexual behaviour, he added: 'This study, and its confluence with other work, strongly suggests that parents need to restrict their children from seeing sexual content in movies at young ages.'
Pippa Smith, of pressure group Safermedia, said the findings were just as relevant to online images as they were to movies.
Here is an activity that is necessary for species survival. Wrapped into it is a combination of beauty, pleasure, and pain (or instinct) that make for an excellent psychic trap.
To overcome this sense of being at the mercy of our own sexuality would be a great advancement for humans.
But these psychologists are almost sure to get it wrong, because that's what they are being paid for. If the powers on earth had any desire to set things straight, we'd be there by now.
I don't care about the pornography. I've seen some it and the quality is uniformly terrible. The quality of portrayal in many movies is much higher, so I think films are the greater problem. But if we try to ban porn on the internet, that will open the door to ban all sorts of other "undesirable" forms of expression.
The handling for the porn industry is to starve it. If we can put people into a spiritual condition where they are no longer interested in that stuff, the industry would collapse. Same is true of drugs, etc. They are all control mechanisms that can only be effectively countered by strengthening the human spirit.