“America’s prison population grew again in 2002 despite a declining crime rate.” So read the lead of an Associated Press report that clacked over the wires yesterday. For a second, one might wonder how more people could end up in prison if fewer crimes are being committed, but the knot isn’t difficult to untangle, especially if twisted around. There is less crime because more criminals are in prison.

That point was marked by James Q. Wilson in the mid-1990s, just as such cities as New York were making a dent in the country’s alarming crime rates. There is a myth, Mr. Wilson said at an Independence Institute policy forum in 1995, that “we have increased dramatically the number of people in prison and gotten nothing for return.”

By way of contrast, Mr. Wilson had this to say: “Coincident with rising prison population there began in 1979–80 a steep reduction in the crime rate as reported by the victimization surveys. This reduction in the crime rate was, I believe, the result of three factors: First, the population was getting older, but this only explains a small part of the decline. Second, there was an increase in the probability of going to prison and this increased the deterrent value of the criminal justice system. Third, the aggregate number of people in prison increased the incapacitation value of the criminal justice system.”

The first factor Mr. Wilson mentioned in 1995 — a declining youth population — has become less relevant as the youth population again swelled in the late 1990s while the crime rate has remained low, as a senior editor at The American Enterprise, Eli Lehrer, pointed out in a 2002 piece in The Weekly Standard. That leaves all the more emphasis on Mr. Wilson’s second two points: the deterrent effect of jail time and the direct effect of locking up criminals.

Mr. Lehrer notes a 1998 study conducted by the Bureau of Justice Statistics, in conjunction with the head of Cambridge University’s Criminology Institute, that found that crime rates fell in America as punishment increased and rose in Britain as punishment decreased. The study’s authors suggested an explanation: “An offender’s risk of being caught, convicted and incarcerated has been rising in the United States but falling in England.” It’s a straightforward way of looking at things. It’s too bad so many people are tied up looking for a more complex explanation.