All that most New Zealanders know of US criminal justice is that they are said to have a higher percentage of their population locked up than us. We are alleged to have the second highest percentage in the 'western' world.

Unfortunately we are not in the same class as the US, despite our incarceration  trajectory. 

We do not have the "wonderful puzzle" of crime rates falling to half the rate 20 years ago. Incarceration appears to be working in the US. 

Here is how the New York Times puzzled over the success of US criminal justice policy on 23 May:

In all regions, the country appears to be safer. The odds of being murdered or robbed are now less than half of what they were in the early 1990s, when violent crime peaked in the United States.  Small towns, especially, are seeing far fewer murders: In cities with populations under 10,000, the number plunged by more than 25 percent last year….

Criminology experts said they were surprised and impressed by the national numbers, issued on Monday by the Federal Bureau of Investigation and based on data from more than 13,000 law-enforcement agencies.  They said the decline nationally in the number of violent crimes, by 5.5 percent, raised the question, at least in some places, of to what extent crime could continue to fall — or at least fall at the same pace as the past two years. Violent crimes fell nearly the same amount in 2009.

“Remarkable,” said James Alan Fox, a criminologist at Northeastern University.  “Given the fact that we have had some healthy declines in recent years, I fully expected that the improvement would slow.  There is only so much air you can squeeze out of a balloon.”

There was no immediate consensus to explain the drop.  But some experts said the figures collided with theories about correlations between crime, unemployment and the number of people in prison.

Take robbery: The nation has endured a devastating economic crisis, but robberies fell 9.5 percent last year, after dropping 8 percent the year before. “Striking,” said Alfred Blumstein, a professor and a criminologist at the Heinz College at Carnegie Mellon University, because it came “at a time when everyone anticipated it could be going up because of the recession.”

For a quick introduction to the quality of discussion you can find on criminal justice matters in the US, check the comments on a blog post on that article here, and bookmark Prof Douglas Berman's home page and the blog "Crime and Consequences" of the Sacramento based Criminal Justice Legal Foundation.

There is to me not much mystery in the different outcome in New Zealand. The US has tackled the gambling preference that distinguishes criminals. Our baffled criminologists persist in thinking that if they are only well-meaning enough, and give enough power to the judges, and corrections officials to be nice to offenders, some day it will pay off.

There is no mystery in the US experience if it is seen as a rational human response to the change in the speed and certainty of consequences for crime. Most of the cahnges of the last 20 years have increased the certainty of detection, conviction and punishment.

A perceived increase in severity may help, but it is much more likely that certainty is what matters.