http://en.wikipedia.org/wiki/PG-13
"Before July 1, 1984, there was a minor trend of cinema straddling the PG and R ratings (per MPAA records of appeals to its decisions in the early 1980s), suggesting a needed middle ground."
Thus, movies predating this had to choose between R and PG. Anything that didn't meet PG criteria automatically got an R rating. A few of the examples above, no doubt, went from R to PG-13 merely because they *could*, not because the first movie or movies in the series were any more violent or explicit.
"Before July 1, 1984, there was a minor trend of cinema straddling the PG and R ratings (per MPAA records of appeals to its decisions in the early 1980s), suggesting a needed middle ground."
Thus, movies predating this had to choose between R and PG. Anything that didn't meet PG criteria automatically got an R rating. A few of the examples above, no doubt, went from R to PG-13 merely because they *could*, not because the first movie or movies in the series were any more violent or explicit.
