New sentencing guidelines have been a key factor. They have reduced judges' discretion in determining who goes to jail and increased the amount of time convicts sentenced to jail spend there. A notable example is the so-called "three-strikes" law, which mandates sentences ranging from 25 years to life for many repeat offenders. Though championed as protecting the public, such sentences have resulted in long confinements for many non-violent offenders, who constitute half of all inmates.

Perhaps the single greatest contributor has been the so-called "war on drugs," which has precipitated a 12-fold increase in the number of incarcerated drug offenders. About 1.5 million Americans are arrested each year for drug offenses, one-third of whom end up in prison. Many are repeat offenders caught with small quantities of relatively innocuous drugs, such as marijuana, a type of criminal activity often referred to as "victimless."

Some sentencing laws seem little less than perverse. For example, in the 1980s, crack cocaine received a great deal of public attention. In response, the U.S. Congress passed legislation imposing a 100 to 1 sentencing ratio for possession of crack cocaine, as compared to its powdered form. That is, someone carrying 5 grams of crack cocaine would get the same sentence as someone carrying 500 grams of powdered cocaine. From a medical point of view, this makes little sense.

The costs of incarceration are high. For example, the state of California spends approximately $9,000 per year for each public school student it educates but over $50,000 per year for each inmate it keeps incarcerated. The proportion of the state budget devoted to imprisonment has been increasing at a rate much faster than that for education. Moreover, despite California's huge prison expenditures, its prisons recently held 140,000 prisoners in facilities designed for only 80,000.

Does prison do any good? This is a surprisingly difficult question to answer. Incarceration certainly works to prevent criminals from committing repeat offenses by removing them from contact with the public. It also provides retribution, satisfying some members of the public that the incarcerated are paying for their crimes. Anyone who visits a prison would be hard pressed to say that it does not represent a powerful form of punishment.

Yet it is difficult to make the case that so-called correctional institutions do much in the way of correcting, reforming, or rehabilitating inmates. The recidivism rate at 3 years post-release is about two-thirds, of which over half end up back in prison. The most important factor in preventing recidivism is not the amount of time people serve in prison, but the age at which they are set free. The older inmates are at the time of their release, the less likely they are to return.