The study, published in Psychological Science, a journal of the Association for Psychological Science, said roughly 85 percent of movies released from 1950 to 2006 contained sexual content, and only 9 percent of sexual content in movies contained messages promoting sexual health. Sexual explicitness of PG-13-rated and R-rated movies has increased over the past decade, researchers also said. Evidence suggests that adolescents' sexual attitudes and behavior are influenced more by movies than by other forms of media, the study 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," Ross O'Hara, who conducted the research with other psychological scientists at Dartmouth College, said.

Richard Ross, professor of student ministry at Southwestern Baptist Theological Seminary, said allowing a middle schooler to watch sex on the big screen is like allowing a toddler to ride in the front seat with no seatbelt, standing up, at rush hour.

Allen Jackson, director of the Youth Ministry Institute at New Orleans Baptist Theological Seminary, likened it to a pregnant woman drinking alcohol.

"There's an enhanced risk at that life stage," Jackson said. "There is an enhanced impact of the images that are viewed."

The effect of media on sexual behavior, the study said, is driven by the acquisition and activation of "sexual scripts." In behavioral psychology, a script is a sequence of expected behavior for a given situation. A higher degree of sexual media exposure, researchers said, has been found to predict more permissive sexual attitudes.

Adolescents sometimes seek out sexual media, possibly to learn scripts, the study said, noting that 57 percent of U.S. adolescents ages 14 to 16 reported using media as a primary source of sexual information.

This particular study examined why movies impact adolescent behavior, looking closely at the role of a personality trait known as sensation seeking, a tendency to seek novel and intense stimulation. Adolescents, experts said, have a predisposition for sensation seeking behavior, which peaks between the ages of 10 and 15.

"It is important to note that sensation seeking arises from both biological and socialization factors ..., which suggests that environmental influences, such as , could affect the development of this trait," the study said.

The study, which was funded by the National Institutes of Health, suggests that exposure to sex in movies may accelerate the normal rise in sensation seeking during adolescence, thereby promoting risky behavior generally, and that it has a lasting influence on risky sexual behaviors in adulthood.

"Given the prevalence of among adolescents, we believe that even small effects of have important implications for adolescents' sexual health," the study authors said. "Our results suggest that restricting adolescents' would delay their sexual debut and also reduce their engagement in risky sexual behaviors later in life."

"One promising approach would involve incorporating media-literacy training into sexual education," the authors said. "A recent intervention showed that a peer-led sexual-media-literacy curriculum increased ninth-grade students' self-efficacy in resisting peer pressure with regard to sexual behavior, reduced their perception of the normative prevalence of sexual activity during adolescence, and improved their attitudes toward abstinence."

Identifying risk factors for early sexual debut and sexual risk taking is an important public health concern, the study said, and delaying adolescents' sexual debut could curb U.S. rates of sexually transmitted infections and instances of unplanned pregnancy.

The study, initially published online July 18, recruited more than 1,200 participants who were from 12 to 14 years old and asked them to report which movies they had seen from several different collections of 50 that were randomly selected.

Participants reported how often they attended church or engaged in religious activities, how many hours of television they watched each day, whether they had a television in their bedroom, and with whom they lived (to determine family structure as intact or divided).

Six years later, the participants were surveyed to find out how old they were when they became sexually active and how risky their sexual behavior was.

"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," O'Hara said. Continued...