Kevin Drum is surely correct when he argues that the U.S. puts way too many people in prison. With the imprisonment rate falling for five straight years and state after state passing major reforms designed to reduce the number of people behind bars, it’s a good time to ask precisely how much de-incarceration reformers need to declare victory.

One possible answer is to strive for a return to the rate of incarceration that prevailed historically until the mid-1970s. Until that point, the imprisonment rate rarely ventured too far in either direction from 0.1% – less than a quarter of the rate we have today. However, that standard was established before the modern feminist movement forced the criminal justice system to take violent crime against women seriously. There are currently a couple hundred thousand men behind bars for raping, assaulting and otherwise terrorizing women. We should not be nostalgic for the era when many or most of them would have gotten away with such crimes. This same point should be borne in mind by anyone who is tempted to look covetously at the low incarceration rates of countries such as India where violence against women almost never results in arrest and prosecution.

Would dropping incarceration to a Western European level be a reasonable goal for the U.S.? Certainly we should move in that direction, but the U.S. is more violent than any of those countries. Our homicide rate is about four times that of the U.K. and about ten times that of France and Germany. It is unrealistic to expect therefore that our incarceration rate will ever drop to their level.

An alternative way to approach the problem is to forget about comparative numerical targets and return to first principles: What is prison for? Typically, we send people to prison for one or all of three reasons (1) to keep the public safe, (2) to provide rehabilitation and (3) to punish people appropriately for things we think are wrong. The current level of incarceration is indefensible on those grounds, but we can strive to return to a level that is.

Kevin notes for example that while we are past the point where increasing incarceration keeps reducing the crime rate, such a point clearly exists, and we could find it again. A further important standard is when overcrowding has eased enough that prisons can again engage in meaningful rehabilitation versus, say, triple bunking inmates in what used to be the basketball court. Finally, when long sentences are again reserved for those who have committed truly serious crimes, we will know that the size of the prison population reflects a commitment to punishing criminals proportionately rather than indiscriminantly.