Update: the data Graeme Edgeler was given didn’t mean what he (reasonably) thought they meant and this analysis is no longer operative. There isn’t good evidence that the law has any substantial beneficial effect. See Nikki Macdonald’s story at Stuff and Graeme’s own post at Public Address.

The usual objection to a “three-strikes” law imposing life sentences without parole, in addition to the objections against severe mandatory minimums, is

It doesn’t work; or

It doesn’t work well enough given the injustice involved; or

There isn’t good enough evidence that it works well enough given the potential for injustice involved.

New Zealand’s version of the law is much less bad than the US versions, but there are still both real problems, and theoretical problems (robbery and aggravated burglary both include crimes of a wide range of severity).

Graeme Edgeler (who is not an enthusiast for the law) has a post at Public Address arguing that there is, at least, evidence of a reduction in subsequent offending by people who receive a first-strike warning, based a mixture of published data and OIA requests.

Here’s his data in tabular form, showing second convictions for offences that would qualify under the three-strikes law. The red cell is ‘first strike’ convictions, the other rows did not count as strikes because the law isn’t retrospective.

Offence Conviction Number Second conviction Number 7/05-6/10 7/05-6/10 6809 7/05-6/10 256 Before 7/10 7/10-6/15 2437 7/10-1/15 300 7/10-6/15 7/10-6/15 5422 7/10-6/15 81

The first and last rows are directly comparable five-year periods. Offences that now qualify as ‘strikes’ are down 20% in the last five-year period; second convictions are down a further 62%. Data in the middle row isn’t as comparable, but there is at least no apparent support for a general reduction in reoffending in the last five-year period.

The overall 20% decrease could easily be explained as part of the long-term trends in crime, but the extra decrease in second-strike offences can’t be. It’s also much larger than could be expected from random variation. The law isn’t keeping violent criminals off the streets, but it does seem to be deterring second offences.

Reasonable people could still oppose the three-strikes law (and Graeme does) but unless we have testable alternative explanations for the large, selective decrease, we should probably be looking at arguments that the law is wrong in principle, not that it’s ineffective.