"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."
The team, reporting in Psychological Science, studied 1228 children aged between 12 and 14 and then analysed their sexual behaviour six years later.
Each teenager identified which popular films of differing classifications they had seen from a random list of 50. Six years later they were asked how old they were when they became sexually active, how many partners they had, how risky their sexual behaviour was and whether they used condoms.
The findings provided a link between exposure to sex on screen and sexual behaviour. Participants also said they tried to mimic love scenes they had seen on screen in the real world.
The researchers also assessed the sexual content of 684 of the biggest grossing films released between 1998 and 2004.
They found some of the most popular films from that time included scenes of a sexual nature, ranging from sexual scenes to heavy kissing.
These include Austin Powers, staring Mike Myers, Notting Hill, with Julia Roberts and Hugh Grant, American Beauty, staring Kevin Spacey and James Bond films such as The World is Not Enough, with Pierce Brosnan as 007.
More than a third of G-rated movies were found to contain "sexual content" compared to more than half of PG films and four in five R-rated movies.
Films with the most sexual content were Summer of Sam, a Spike Lee crime film about a series of 1970s murders in New York (323 seconds), 40 Days of 40 Nights, staring Josh Hartnett who tries to stay celibate during Lent (207 seconds) and American Pie, about a group of high school students trying to lose their virginity (206 seconds).
Even children’s films were found to have sexual content such the G-rated the Princess Diaries (42 seconds).
Most of the recent films did not portray safe sex, with little mention of using contraception.
Dr O'Hara said that the combination of sexually explicit films and adolescence had a profound impact on their behaviour.
He found that the “wild hormonal surges of adolescence” made cautious thinking amongst teenagers more difficult.
He said that while more than half of adolescents use movies and the media as their “greatest source of sexual information” many could not differentiate between what they saw on a screen and what they confronted in real life.
Dr O'Hara added: “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.”
A previous survey of films from 1950 to 2006 found that 84 per cent of movies contain sexual content.