How did indeterminate sentences link release from prison to an offender's reform?

An indeterminate sentence is a term of imprisonment with no fixed duration, typically defined by a prescribed minimum and maximum period. This sentencing structure links an offender's release directly to their perceived reform by making the length of incarceration dependent on the progress they demonstrate while in custody.123

Under this system, release is not determined by a judicially fixed date but by the evaluation of prison authorities, who assess factors such as the offender's conduct, attitude, and participation in training or treatment programs. The mechanism functions as follows:13

• Incentive for Reform: The uncertainty of the release date provides a powerful incentive for inmates to improve their behavior, engage in vocational education, and demonstrate that they are ready to return to society as law-abiding citizens.32
• Disincentive for Misbehavior: Conversely, the prospect of continued detention serves as a deterrent against poor conduct or failure to adhere to institutional rules.2
• Discretionary Assessment: Parole boards and prison staff monitor the offender's progress to determine their suitability for release, effectively using the duration of the sentence as a tool to ensure the offender has achieved a sufficient level of rehabilitation.132