London ? Youngsters who watch movies in which actors smoke a lot are three times more likely to take up the habit than those exposed to less smoking on-screen, a new study of American adolescents suggests.

The study, published today on the Web site of The Lancet medical journal, provides the strongest evidence to date that smoking depicted in movies encourages adolescents to start smoking, according to some experts. Others said they remained unconvinced.

Many studies have linked smoking in films with increased adolescent smoking, but this is the first to assess children before they start smoking and track them over time.

The investigators concluded that 52 percent of the youngsters in the study who smoked started entirely because of seeing movie stars smoke on screen.

“This effect is stronger than the effect of traditional cigarette advertising and promotion, which accounts for only 34 percent of new experimentation,” said Stanton Glantz, a professor of medicine at the Institute for Health Policy Studies at the University of California, San Francisco, who was not connected with the research.

However, Paul Levinson, a media theorist at Fordham University in New York, noted there were many reasons people start smoking and the study could not accurately determine how important each factor is.

“It’s the kind of thing we should be looking at but … the fact that two things seem to be intertwined doesn’t mean that the first causes the second,” said Levinson, who was not involved in the study. “What we really need is some kind of experimental study where there’s a controlled group.”

The Motion Picture Association of America, which rates movies and represents the movie industry, had no immediate comment.