Low crime rate, high imprisonment rate – what does this tell us?

You may have heard the good news: New Zealand’s crime rate has been dropping. In fact, it has dropped to the lowest it has been in 29 years.

Following the release of our report Unlocking Prisons: How we can improve New Zealand’s prison system, it was put to us that because New Zealand’s crime rate has dropped, and we have a high imprisonment rate, our prison system must be working.

The simple answer to this, with a social phenomenon as complex as criminal offending, is that there is no evidence to suggest that a high degree of imprisonment is responsible for our low crime rate. What’s more, it is incorrect to suggest that there is even a correlation between the two.

There are no simple explanations for the decline of crime in New Zealand

Crime rates, and the causes of changes to them, are a contentious area of research.  It’s worth pointing out that falling crime rates are a global trend in the developed world, and this pattern is being seen in a number of countries across the world, including Australia, the United KingdomCanada and the United States.

It is not possible to attribute the drop to any one thing. JustSpeak recognises that any number of factors may be responsible. We support, for example,  some research which goes beyond correlating one factor with another and suggests that causes of the drop might include better policing practices, such as the effectiveness – especially for young people – of Police increasingly using warnings instead of prosecuting for offences.

No evidence of causation between the imprisonment rate and the crime rate

“Correlation does not equal cauation” is a scientific principle that basically means a correlation between two different variables does not necesssarily imply that one causes the other. Sometimes there is a correlation between two variables, but scientists are very wary to draw such correlations unless there is sufficient evidence that the variables are actually linked.

Great care needs to be taken before inferring that one variable is caused by another.

As noted in the Economist,

“ [the high imprisonment rate] has obviously had some effect—a young man in prison cannot steal your car—but if tough prison sentences were the cause, crime would not be falling in the Netherlands and Germany, which have reduced their prison populations. New York’s prison population has fallen by a quarter since 1999, yet its crime rate has dropped faster than that of many other cities”.

Or, as we’ve pointed out in our report, thinking that imposing severe punishments like imprisonment prevents the commission of offences is based on a rational-act model of thinking, assuming that people weigh up the costs and benefits of a particular course of action at the time of every decision.

Research indicates that increases in the severity of punishment do not produce a corresponding increase in the general deterrent effect (deterring society in general). Furthermore, at an individual level, research shows that imprisonment has, at best, no effect on the rate of reoffending and is often criminogenic, resulting in a greater rate of recidivism by imprisoned offenders compared with offenders who received a different sentencing outcome.

No correlation between the crime rate and the imprisonment rate

Not only is there no causation between the crime rate and the imprisonment rate, there actually isn’t even a correlation either.

As our friends at Rethinking Crime and Punishment have written,

“There is no correlation anywhere in the world between the imprisonment rate and the crime rate. The imprisonment rate is not a measure of crime. It’s a measure of the consumption of punishment”.

The below graph demonstrates that the number of people convicted of offences does not correlate to the number of people in prisons.


Unlocking Prisons: an alternative inference?

In the report, we set out the ways in which we think our prison system is failing. We hope people will consider our findings, especially given the extensive research on which we draw, before suggesting that our dropping crime rates mean prisons are an effective way to deal with and reduce crime.

Crime and punishment are emotive subjects. We know not everyone will agree with the conclusions in our report, but our aim is to ensure that everyone is as informed as possible about what is—and isn’t—working to make New Zealand safe.