The key thing I think about is budget, genre, perception. Budget: You can't write an R-rated Transformers, or The Avengers. R-rated movies have a generally more consistent box office performance, usually in the 30-50mil range (see The Grey). You don't want to put F-bombs or nude women in a $220mil blockbuster, because you're cutting a massive portion of the studio's potential audience right off the bat. In fact, for a movie that is going at all for the family market, you don't want even the token "one use" of an F-word that's allowed in a PG-13. Avatar used it, but Avatar was a bit more adult-oriented. Parents could get upset about it and tell their friends not to take their children. Genre: A PG-13 horror movie is generally taken less seriously than an R-rated horror. A PG-13 war movie is almost a no-go, but that could change if you found a story that would draw family audiences (see War Horse, only... maybe less sappy and childish? That movie would have done better if it focused on adult sensibilities as much as children). On the other hand, Taken (PG-13) did much better than The Grey (R), and while some of that was just the difference in concept, a good part of it was also because The Grey cut off teenage boys from seeing a badass movie about Liam Neeson fighting wolves. Perception: When movies are meant to be taken seriously - like... Academy Awards kind of seriously - they're generally rated R. Obviously, there are always serious PG-13 awards-contenders, but if you're writing a movie that has little appeal to teens (such as a historic biopic or a ballet horror or a movie about a journalist interviewing a scandalized president), if you want it to be taken seriously by audiences and critics alike, the R-rating is like a sign on the poster that says "This movie deals with some serious shit." But as Scott said: all practical considerations aside, write what's best for your story. -JMT

All the Matrix movies are R rated, btw. They all did pretty good box office.

Thanks guys, this is very useful stuff! I'm going to mull this over for while. Cheers.