Increases in crime-prone groups in the population were not found to be associated with increased prison admission rates. Admission rates for the demographically disadvantaged groups changed over time, and the pattern of changes varied depending upon the group examined. There was a relatively small growth in the prison population from 1975 to 1980, and over eighty-five percent of the growth could be explained by demographic changes. In contrast, from 1980 to 1983 there was substantial growth in the prison population, yet only 6.7 percent of the growth could be attributed to demographic shifts. Possible reasons for the pattern of changes found for the different age, race, and sex groups and for the large admission rate increases in the early 1980s are discussed. 16 references. (Author abstract)