A message picture is a motion picture that, in addition to or instead of being for entertainment, intends to communicate a certain message or ideal about society. Message pictures usually present the message they want to deliver in the form of a morality play, and are usually serious (often somber) works. However, not all message pictures are 100% serious, and there are also films spoofing the genre (Sullivan's Travels, for example).
Many people use the term to indicate movies that put forth a message or idea they don't like; this is because many message pictures portray a socially liberal or progressive viewpoint, and message pictures depicting a more conservative viewpoint are rare.
Dore Schary was famous for his message pictures at Metro-Goldwyn-Mayer during the late 1940s and early 1950s. Among these were The Next Voice You Hear , Asphalt Jungle , and Blackboard Jungle. Other famous message pictures by other parties include Guess Who's Coming to Dinner? and In the Heat of the Night.
Needed: More examples of the genre, NPOV checking.