Blockbuster movies are less likely to portray smokers than they have in the past, according to a new study. What's more, this decline in on-screen smoking may have occurred in tandem with a drop in the number of adolescents who have lit up in real life.

Teenage smoking and the number of smoking scenes in movies have declined, according to a study.

While the study can't prove that one is related to the other, the findings would seem to support what critics have long said: Smoking by glamorous (or even not-so-glamorous) people on the silver screen is like free advertising for cigarettes.

A second study, also published in a letter in this week's Journal of the American Medical Association, suggests that the portrayal of gun use has slightly declined in children's movies as well.

"Reducing smoking in movies probably helped to reduce rates of smoking in kids," said study author Dr. James D. Sargent, a professor of pediatrics at Dartmouth Medical School and the codirector of the Cancer Control Research Program at Norris Cotton Cancer Center, both in Lebanon, New Hampshire. "We are on the right track aiming at movies, yet half of movies still contain smoking and more work needs to be done." Health.com: Quitting smoking can help halt heartburn

About one-third to one-half of adolescents who begin smoking may do so because they see smoking in the movies, some research suggests, and the tobacco industry has spent big bucks in the past to make sure cigarettes appeared in popular movies, according to Smoke Free Movies, a University of California San Francisco project. For years, public health groups have encouraged the movie industry to voluntarily reduce depictions of smoking.

Their efforts may have paid off, at least in younger teens. In the study, researchers looked at smoking scenes in the top 25 highest-grossing movies from 1990 through 2007. Each time a movie character handled or used tobacco, or even when tobacco was seen in the background, it was considered a smoking scene. Health.com: My smoker's cough turned out to be emphysema

In 1990, there were about 3.5 smoking scenes per box-office hit. By contrast, there were only 0.23 instances of smoking in the highest-grossing films of 2007. At the same time, there was a corresponding decrease among eighth graders who smoked. Smoking peaked in 1996 with 21 percent of eighth graders saying they had used cigarettes (defined as having smoked in the previous 30-day period) and dropped to 7.1 percent in 2007, the study showed. Overall, smoking in eighth graders decreased by an average of 1.3 percentage points each year.

"It's great to see smoking on the decline; it's the leading preventable cause of death, and most of those who take up the habit nowadays do so as teens," said Jeff Stier, the associate director of the American Council on Science and Health, a New York City-based group that publicizes the health risks of smoking. Health.com: 5 Celebrities who had (or are at risk of) the lung disease COPD

However, the picture in older teens is more complicated. About 27 percent of children in grades 9 through 12 smoked cigarettes in 1991, and that increased to 36 percent in 1997. There was a steep decline after that, hitting about 22 percent in 2003. Since then, teen smoking has plateaued or only declined slightly, and 20 percent of U.S. high school students were cigarette smokers in 2007--approximately 19 percent of females and 21 percent of males.

In the second study, researchers looked at gun violence in G- and PG-rated films that were marketed to children from 2003 to 2007. Firearms are commonly seen in children's movies, but there was a decrease in the number of characters handling guns in that time period, when compared to data from a similar study conducted from 1995 to 2002.

Of 125 G- and PG-rated movies from 2003 to 2007, 27 percent included characters with firearms. Of these, 99 percent were adults, one was a child, and 90% were male. More than half of those characters with weapons were police officers, security guards, or other members of law enforcement, and 36 percent were criminals. Twenty-seven of these characters shot their weapons, with 59 percent aiming at a human. The movies rarely showed the consequences of gunfire, according to the research team led by Jon Eric Tongren of the Centers for Disease Control and Prevention, in Atlanta, Georgia.

Dr. Victor C. Strasburger, a professor of pediatrics at the University of New Mexico School of Medicine, in Albuquerque, New Mexico, would like the Motion Picture Association of America (MPAA) to give an automatic R-rating to movies that show smoking or violence. The MPAA is the primary group responsible for rating films in the United States.

Witnessing smoking scenes in movies may be the leading factor associated with smoking initiation among youth, Strasburger wrote in a JAMA editorial, which is based on research by his group. Health.com: Can't stop coughing? 8 causes of chronic cough

"[The MPAA] should have a pediatrician and child psychologist on board to help with ratings," Strasburger said. "They don't rate heavily enough for violence. Violence in mainstream movies is out of control and Hollywood needs to accept more responsibility."

If the movie industry reigned in the violence, "I think there would be a decrease in violence in society," said Strasburger. "It's not just guns -- it is interpersonal violence. So the average 4-year-old boy learns very early that it is OK to punch somebody out if he disagrees because that is what he sees in movies."

However, Stier urged caution "about assuming that there's a clear, causal connection between movie smoking and real-life smoking-- the two might well rise and fall together simply because of broader societal trends stigmatizing or valorizing cigarettes."

Some studies that have shown a connection between movie smoking and behavior were "recall" studies in which smokers were more likely to say they remembered seeing a lot of smoking in movies. But then, people who like doing something may be more likely to recall its depiction, Stier said.

"I wouldn't want to see a belief in strict, causal connection here lead to a law banning smoking in films," he said. "But the more people choose to avoid or quit the habit, and the less artists choose to glorify it, the better."

Strasburger said that parents should know that even G- and PG-rated movies may portray drugs or violence, and PG-13 and R-rated movies may have an abundance of violence, sex, or drug use. "Go to see movies with your kids and talk about the content," he suggested. The same vigilance should be applied to TV shows and video games, he added.

Stanton Glantz, Ph.D., the UCSF professor who launched Smoke Free Movies, says that the "important thing about this paper is that it is more evidence that smoking in the movies causes kids to smoke."

His group's data suggest that smoking in movies peaked three years ago and has declined by 30 percent since then, although he says there's plenty of room for improvement.

"The fact that movie smoking remains high is one reason youth smoking did not drop more," he says. Both the World Health Organization and the American Medical Association Alliance recently said that movies with smoking should be R-rated, due to the link to youth smoking.

If you want to know what movies do or do not portray smoking, check out this list of films (including current releases and DVDs) provided by Smoke Free Movies.

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All About Smoking • Movies • Journal of the American Medical Association