Wednesday 23 September 2009

Three strikes law for NZ?

The ACT party put forward a bill proposing a three-strikes rule that, in its initial form, eliminated marginal deterrence at the third strike: 25 year sentence on third strike regardless of the offense. Of course, we know this can induce a severity shift among offenses on the strike list and provides little inducement for criminals to leave witnesses hanging about.

Kiwiblog reports that negotiations between ACT and National has led to a bill that National would support at second reading which would require judges to impose the maximum penalty on a third strike. Where Farrar emphasizes the fairness aspects of not having relatively minor offenses draw a 25-year sentence, I'm far more interested in the maintenance of marginal deterrence. Having the third strike draw the maximum sentence for that offense does a pretty good job: there's no reason to expect a severity shift among offenses since they all still draw differential penalties. Perhaps we could expect such a shift within a particular offense, but that can't have too large an effect.

Joanna Shepherd's work shows rather convincingly that strike-type legislation in California was remarkably successful in deterring even first strikes. The approach here proposed would avoid some of the worse potential consequences of the California law (severity shift, excessive prison costs) while keeping a decent chunk of the benefits.

We'll have to wait for the final bill that comes through. There are lots of ways of maintaining marginal deterrence:
  • Second strike: maximum penalty; Third strike: some multiple of the maximum
  • Second strike: minimum penalty is no less than 75th percentile of penalties awarded on first strike for that offense; Third strike: maximum penalty
  • If you were paroled after 1/3 of your sentence on the prior strike, adding 2/3 to your next-strike sentence

David Friedman's work shows reasonably convincingly that you need to have a higher penalty for a second offence than for a first one just to maintain deterrence; maintaining marginal deterrence then also requires that there aren't flat portions of the expected punishment curve across severity of offence. Lots of ways of achieving that, but a mandatory 25-year sentence for any third-strike isn't one of them.

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