Compared to 31 jurisdictions in Europe, the United States and Australia, New Zealand's prisons have the highest percentage of sentenced prisoners convicted of violent and sexual offences, a research paper by Department of Corrections policy adviser Marcus Boomen says.

The paper, "Where New Zealand stands internationally: A comparison of offence profiles and recidivism rates" is published in the latest issue of the New Zealand Corrections Journal. It says New Zealand is "unusually skewed" in terms of sexual and violent offenders in its prison population.

Mr Boomen says with 197 prisoners per 100,000 population in 2015, New Zealand's incarceration rate far surpasses those of Western European countries.

"While English-speaking jurisdictions tend to have higher imprisonment rates than continental Europe, New Zealand stands out further with an imprisonment rate surpassing many of those in Eastern Europe also. By 2017, our prison population was 219 prisoners per 100,000, significantly higher than Australia (162) or England/Wales (145)."

He says comparative analysis of the factors that can influence the prison population is necessary for an understanding of why New Zealand's prison population is higher than other developed jurisdictions.

Looking at one factor, the prison offence profile, he analyses information from 32 jurisdictions, including New Zealand. Information on the proportion of prisoners by lead offence type at 1 November 2015 shows that New Zealand had the highest proportion of violent offenders - who made up 18.5% of prisoners. New Zealand was followed by Northern Ireland (18.4%) and Australia (17.7%). The median for the 32 countries was 6.6% of prisoners.

New Zealand also led in sexual offences, with 25.2% of prisoners whose lead offence was a sexual offence. Next was Norway (17.9%) and England and Wales (15.8%). The median for all countries surveyed was 7.3%.

"When these categories were combined with that for homicide (and related offences) into one for all 'interpersonal violence', New Zealand still recorded the highest proportion of prisoners of this type." he says.

In New Zealand, 53.6% of prisoners' lead offence was 'interpersonal violence', followed by 46.6% in Albania and 46% in the United States. The median for the 32 countries was 33.2%.

Per capita

"Of all 33 jurisdictions, New Zealand has high proportions per capita of people imprisoned for sexual and violence offences. Only in regard to homicide and related offences do we rank lower, at thirteenth place with 17.3 offenders per 100,000," Mr Boomen says.

On a per capita basis by lead offence type, Mr Boomen's research shows the following position at 1 November 2015:

Possible reasons

Mr Boomen says the data do not explain why New Zealand has such high proportions of sexual and violent offenders. He says to shed further light on these findings, the mix of offence types by length of imposed sentence and throughput also needs to be examined.

He says the most notable feature is that those sentenced for sexual offending are mostly serving relatively long sentences. Of all sexual offenders in prison at that time, 63% were sentenced to more than five years.

"Given such a high proportion of sexual offenders are in New Zealand prisons, and the fact that they are mostly serving very long sentences, two hypotheses present: that similar offenders in other jurisdictions spend less time in prison, and/or New Zealand has larger numbers of these offenders entering prison.

"Unfortunately, we do not have the data to answer these questions, however, these results cast some doubt on the idea that tough New Zealand sentencing practices alone are the cause. For this to be true sexual offenders serious enough to get more than five years in New Zealand would have to be diverted from prison in other jurisdictions, or would serve a sentence so short they would churn through the system without resulting in high prisoner numbers. More research is needed to answer these questions."