WASHINGTON — You don’t need to be a pacifist to notice that American motion pictures have gotten way more violent, and that younger and younger audiences are seeing more intense violence on the big screen. But for skeptics, a new study, published online in the Journal of Adolescent Health, offers some validation.

Researchers from the Annenberg Public Policy Center of the University of Pennsylvania coded each year’s top-grossing 30 films from 1950 to 2006 to gauge the extent and intensity of sexual content and violence. They then sought to discern trends within ratings categories, and the migration of sexual and violent content into movies intended for the broadest circulation — G, PG and PG-13 movies.

The sexual content of PG and R movies started accelerating in the late 1960s, when the Motion Picture Association of America’s ratings system was instituted. It stabilized in the late 1970s and even declined somewhat after that. Since then, movies bearing PG and PG-13 ratings have not become more sexually explicit, the study found.

Not so with violent content. In fact, it exploded across the PG-and-up ratings categories, cascading heavily into a new category introduced in 1984 — PG-13. And as movies in the PG-13 category surged — in recent years, they have come to represent about half of all top-grossing movies — so did the violence in them. From 2001 to 2006, “ratings creep” resulted in PG-13 movies that had more violence and more intense violence than did R-rated movies, compared with the 1977-1984 period.

The trend worries researchers, who point to evidence suggesting that youth exposed to extensive media violence are more likely to engage in aggressive behavior, that they are less likely to reject violence as a means of solving disputes and have less empathy for victims of violence.