Researchers who study the effects of the mass media typically focus on the immediate, short-term effects of a particular program or movie. However, many of the effects of media exposure occur over the long term, with repeated exposure over time. One important area in which there is extensive evidence for cumulative effects is media violence.

By the 1970s, scholars concurred that early childhood exposure to media violence caused children to be more aggressive. As Leonard Eron and his associates wrote (1972, p. 262), "the weight of evidence… supports the theory that during a critical period in a boy's development, regular viewing and liking of violent television lead to the formation of a more aggressive life style." The consensus of scholarly opinion in this area prompted the U.S. Surgeon General to issue a warning in 1972 about the cumulative effects of viewing violence in the media.

While...