WASHINGTON — The MPAA is marking the 50th anniversary of its film ratings by releasing data on how movies have been rated over the last half-century.

The R rating has been placed on by far and away the majority of the movies since then, more than half of the almost 30,000 titles that have been given classifications.

The breakdown: Since 1968, the first year of the rating classifications, there have been 17,202 movies rated R, 5,578 rated M/GP/PG, 4,913 rated PG-13 and 1,574 rated G. Just 524 movies have been rated X or NC-17, reflecting the reluctance of exhibitors to carry those titles.

The MPAA also said that over the years, 1.4 percent (or 428 titles) of the ratings have been appealed, and 0.6 percent (or 165 movies) have had their rating overturned. The MPAA’s complete report is here. The organization also released a digital archive of documents, including such things as press releases and letters when it was formed, and copies of the pre-rating Motion Picture Code.

Joan Graves, the chair of the Classification and Rating Administration and MPAA senior vice president, also answers commonly asked questions in a series of videos.

The trade association also released the results of a survey, conducted by Nielsen, of 1,559 parents of children between the ages of 7 and 16. It showed that 59 percent strongly and 36 percent somewhat agree that the ratings are helpful tools. There also was substantial agreement that the descriptors for the ratings were helpful tools for parents. The MPAA also reviews advertising content and said that it has overseen about 68,000 pieces of material over the past year, including 15,835 trailers.

MPAA Chairman Charles Rivkin said in a statement, “Given the extraordinary changes in our culture, entertainment, and society over the last 50 years, this anniversary feels particularly hard-earned and special. We could point to many factors behind the ratings’ success, but the clearest one of all comes directly from our founding mission: to maintain the trust and confidence of American parents.”

The ratings were announced on Nov. 1, 1968, by then-chairman Jack Valenti against threats of government censorship, particularly in local communities. The last state censorship board was disbanded in 1981.

Valenti, who became the head of the MPAA in 1966, was quickly faced with the controversies over “Who’s Afraid of Virginia Woolf?” and “Blowup.” Graves said that “the fact that he was able to convince all those different parties, the makers of films, the exhibitors of films, and the censors of films, and the church groups to take a chance on this, seems even more remarkable today I think than it was then.”

Before the ratings system was adopted, the industry operated under the Production Code, or Hays Code. The MPAA report goes into great detail about the history of that code, including some of the restrictions that “seem ridiculous today.” Among them: One foot on the floor in love scenes, the depiction of childbirth as “painful,” and scenes of toilets. “Psycho,” made in 1960, was the first film to show a flushing toilet, according to the report.

The current ratings have evolved over time. The original ratings were G, M, R and X. In 1984, a “PG-13” rating was established to find a middle ground between PG and R, after controversy over some of the violence depicted in “Indiana Jones and the Temple of Doom.” The MPAA said that the introduction of the PG-13 rating has reduced the number of studio and filmmaker appeals of their rating classifications.

The X rating originally went to mainstream movies, including “Midnight Cowboy,” released in 1969 and the only best picture winner to be rated X. The adult film industry began using the descriptor, though, and most studios and exhibitors steered clear of earning the classification. The MPAA replaced the rating with NC-17 in 1990, but that too has been met with reluctance on the part of exhibitors.