In the past few weeks, the health journal Pediatrics has published two studies concerning violence in movies. "Gun Violence Trends in Movies" found that gun violence in American PG-13 movies has increased over the past twenty years to the point where there is now as much gun violence in PG-13 films as in R films. It is well established that the mere presence of weapons increases aggression, and this study suggests that both the increased presence and use of guns in PG-13 films are examples of ratings creep.
"Violent Film Characters’ Portrayal of Alcohol, Sex, and Tobacco-Related Behaviors" considered the occurrence of violence associated with other rating affecting behaviours such as sexual activity or alcohol use. This study found that PG-13 movies and R movies have equal frequency of violence combined with other mature or risky activities. Again, ratings creep is suggested.
Both studies note that producers aim for the PG-13 rating to maximize revenue, and imply that there is no significant difference between a PG-13 rating and an R rating. The studies also note that exposure to violence increases aggression, and provides "scripts" for impressionable youth to emulate. Their conclusions are that the level of violence in PG-13 movies may be harmful to teens, and that the MPAA may not be effective in reducing teens' exposure to violence.
Canadian film ratings tend to give more weight to violence than American ratings, but not so much that the studies' outcomes would be significantly different here.
Given the publication, it's no surprise that the studies are concerned with the effects of violence on children. However the logic of most film ratings systems is that adults should be allowed to see as much of anything they wish and that only children need to be protected. Unfortunately this approach ignores the reality that viewing violence affects everyone, not just children. While it is reasonable to be concerned that children now have more exposure to violence in movies, we should also be concerned that everyone is exposed to more violence in movies.
Limiting movie content through censorship is probably no longer feasible. What is possible, and perhaps desirable, is that the potential social harm of violence in films be recognized. It could be treated much the same way alcohol or gambling is - as something to be enjoyed in moderation. I'm not sure how one determines what moderate consumption of violence is, but any limits would be better than none.