The Motion Picture Association of America announced changes to its movie rating system Tuesday, saying it wants to help parents make informed decisions at the multiplex.

The new system, rolled out as the "Check the Box" campaign, will include prominent descriptions explaining why a movie received its rating. Films that might previously have been stamped PG-13 with a sentence beneath the rating will now feature those same descriptions in large type next to the ratings code.

The changes announced by MPAA CEO Christopher Dodd in Las Vegas on Tuesday come in the aftermath of explosions at the Boston Marathon and recent shooting rampages, though the former U.S. senator did not address such examples directly.

The White House has called on the movie industry help parents monitor violence in media since the elementary school in Newtown, Conn., the state Dodd represented for 30 years as a Democrat until 2011. And in a sweeping proposal this year, President Barack Obama asked specifically for a stricter rating system.

Dodd announced the industry's plan at the annual movie-theater convention CinemaCon and spoke generally about the need to help parents "so they can make the best choices about what movies are right for their children to watch."

The MPAA began issuing ratings descriptions for every film rated PG or higher in 1990. Those descriptions will now feature more prominently.

One example read, "An intense scene of war violence, some images of carnage, brief strong violence."

"We're changing the way they're presented so that they're easier to read," MPAA spokeswoman Kate Bedingfield said.

Some observers had hoped Dodd might use his keynote address to signal to the industry that the MPAA would begin assigning R ratings to all hyper-violent movies, potentially limiting their audience and quashing their box office appeal.

Conservative groups have for years accused the MPAA of "ratings creep," a ratcheting down of ratings in the interest of profits, so that material once considered a PG-13 now gets a PG and what once was an R is now a PG-13.

"I am not moved," said Tim Winter, the president of nonpartisan Parents Television Council "I think this is a distinction without a difference. A cynical view of the announcement today is, How can the MPAA protect themselves and continue a toxic level of violence, especially in PG-13 movies, while providing themselves cover from all the scrutiny?"

Gun control advocacy groups greeted the announcement with a shrug.

The MPAA will bolster the campaign with a public service announcement that will be shown before movies and posters that will appear in theaters across the country. It will also update its green pre-trailer screens to clarify that the trailer is approved for the audience viewing the main feature.