How did prisons change from holding people before trial to punishing people after conviction?

Until the late 18th century, prisons were primarily used for the temporary confinement of debtors, individuals awaiting trial, or convicts awaiting sentencing—which typically involved death or transportation overseas rather than imprisonment. Prisons were not originally designed as places of punishment themselves; imprisonment was rarely imposed as a sentence, except for minor crimes.1

The shift toward using prisons as the primary site of punishment occurred as humanitarian ideals grew and the use of capital and corporal punishment began to decline. By the early 19th century, the concept of the "penitentiary" emerged, framing the prison as a place for both punishment and personal reform. This transition led to the development of structured systems, such as the Pennsylvania system (emphasizing solitary confinement) and the Auburn system (emphasizing silent group labor), both intended to prevent the negative influence of inmates on one another.123

Modern penology further evolved to prioritize personal reform through methods like the mark system and reformatories, which introduced vocational education, indeterminate sentences, and parole. Today, imprisonment serves multiple purposes, including deterrence, retribution, public protection, and rehabilitation.34