Incarceration is the etention of a person in jail or prison. People are most commonly incarcerated upon suspicion or conviction of committing a crime. Incarceration rates, when measured by the United Nations, are considered distinct and separate from the imprisonment of political prisoners and others not charged with a specific crime. Historically, the frequency of imprisonment, its duration, and severity have varied considerably. There has also been much debate about the motives for incarceration, its effectiveness and fairness, as well as debate regarding the related questions about the nature and etiology of criminal behavior.