The Census Bureau counts people where they live and sleep most of the time, and it counts incarcerated people at the prison. Most people in prison cannot vote, and most will return home. Their numbers still add to the population of the district holding the prison, so that district needs fewer actual voters than others.
The effect is sharpest in local races, where a single prison can make up a large share of a small district. Because Black and Latino people are incarcerated at higher rates, the shift moves representation away from their home neighborhoods. The Prison Policy Initiative has tracked the issue since the early 2000s.
New York and Maryland passed the first laws to end the practice in 2010, counting people at their home addresses for redistricting. By 2025, 14 states had acted, including California, Illinois, Virginia and Washington. The Census Bureau kept its prison counting rule for the 2020 census, so states must adjust the data themselves.