How do swear words factor in to a movie’s MPAA rating?

Posted under Ask the Mediatrician.

Q: I let my 13-year-old watch the movie “Once” and I didn’t understand why it was rated R; there was some bad language but no irresponsible display of risky behavior like so many PG-13 movies I’ve seen. How do movie ratings work?
Answered by Dr. Michael Rich, The Mediatrician