“TOO many Americans go to too many prisons for far too long, and for no truly good law-enforcement reason.” The person who said that was neither a defence lawyer, nor a prisoners’-rights advocate, nor a European looking down his nose across the Atlantic. It was instead America’s top law-enforcement official, Eric Holder, the attorney general. On Monday Mr Holder announced several changes to federal prison policy, the most important of which was that federal prosecutors will no longer charge low-level, non-violent drug offenders with crimes that trigger “draconian” mandatory-minimum sentences. But how did America’s prison population become so unmanageably huge? America has around 5% of the world’s population, and 25% of its prisoners. Roughly one in every 107 American adults is behind bars, a rate nearly five times that of Britain, seven times that of France and 24 times that of India. Its prison population has more than tripled since 1980. The growth rate has been even faster in the federal prison system: from around 24,000—its level, more or less, from the 1940s until the early 1980s—to more than 219,000 today.

Probably the biggest driver of this growth has been ever-harsher drug penalties. In response to the crack epidemic of the 1980s, Congress and state legislatures began passing laws that meted out mandatory-minimum sentences for drug-related crimes. These were intended to help nab major traffickers, but the sentences were triggered by the possession of tiny quantities of drugs: five grams of crack, for instance, resulted in a mandatory-minimum sentence of five years. Conspiracy laws made everyone involved in a drug-running operation legally liable for all of the operation’s activities: a child hired for a few dollars a day to act as a lookout at the door of a crack house was on the hook for all the drugs sold in that house and all the crimes associated with their sale. These sorts of laws kept America’s prison population growing even as its crime rate declined.

The tide began to turn around ten years ago when, in classic Nixon-to-China fashion, hang-’em-high Texas passed a law sending low-level, non-violent drug offenders to treatment rather than prison. The reform movement gained steam when the financial crisis hit: incarcerating people is expensive. Since 2007 more than half of America’s states have enacted some form of criminal-justice reform, and since 2008 the number of Americans behind bars has dipped slightly. How much of a dent Mr Holder’s policy shift will make remains unclear: it applies only to federal prisons, and around 90% of incarcerated Americans are in state and local lockups. But it’s a good start.