When stability masks change: pretrial detention and changes in county jail use, 1999–2019

Despite growing attention to bail reform and pretrial practices, less is known about how the role of U.S. county jails has changed over time. This study analyzes long-run changes in jail use between 1999 and 2019, with particular attention to the expansion of pretrial detention. Drawing on county-level data from the Census of Jails, the analysis applies a decomposition framework that separates national change into within-county institutional shifts and population redistribution across counties. The results indicate that while overall jail incarceration remained largely stable, pretrial detention increased substantially in both rate and share. Nearly all observed change reflects within-county shifts in detention practices rather than compositional change driven by population movement. Similar patterns are evident across metropolitan and nonmetropolitan counties. These findings highlight the importance of treating jails as dynamic institutions whose functions evolve over time and demonstrate how stability in aggregate incarceration can coexist with significant institutional transformation.