During the past 30 years, the US prison population has expanded dramatically. Early research focused primarily on economic explanations for the US prison population explosion. More recent explanations for this trend have focused on population growth. PPC research by Professor Karen Heimer further extends previous work by incorporating political and public policy literature. It argues that partisan politics regarding punishment influence levels of electoral competition for legislators. To test this hypothesis, researchers use annual state-level imprisonment data from the years 1978 to 1998. They find that the level and timing of electoral competition influences the effect of Republican state legislative strength on prison admissions. The study concludes that a more nuanced understanding of the interplay between state-level partisan politics and imprisonment is needed.