Why haven't we noticed? Maybe it's the clamour from media darling Garth McVicar and his misnamed Sensible Sentencing Trust, from whence came the discredited former Act MP David Garrett and the push for "three strikes".
In his 2006 book The Great American Crime Decline, law professor Franklin Zimring observes that the US was well into its dramatic 1990s "crime decline" before anybody noticed, and that, remarkably, the early years of falling crime rates were "the most pessimistic about crime in American history".
When it became obvious that the pessimism was misplaced, the hotly debated question was then, "Who gets the credit?" On the West Coast, proponents of California's 1994 three strikes law with its mandatory 25-year prison sentences were claiming victory for tough prison sentences.
But on the East Coast, New York's much-vaunted "broken windows" zero-tolerance policing was winning acclaim for the most dramatic crime fall in the US - twice the national average.
So what was it? Harsh penal laws? Improved police tactics? Legalised abortion? Or the end of the crack cocaine epidemic?
Zimring writes that the 1990s were "a cascade of best-case outcomes - high levels of incarceration, a drop in the proportion of the population in high-risk youth categories, and unprecedented prosperity for the same nine years that crime declined".
But his analysis shows that even that confluence of population, economic, and incarceration trends could only account for half the decline. "The available evidence stops well short of a complete or precise account of the causes of decline."
That hasn't stopped the extravagant claims, especially from fans of imprisonment.
One problem with the imprisonment theory is that in the US, "massive doses of increased incarceration had been administered throughout the 1970s and 1980s with no consistent and visible impact on crime".
The other is Canada, which saw similar falls in crime without the imprisonment boom.
But even if locking up more people for longer periods reduced crime, California's prison crisis suggests that excessively punitive approach comes at too high a cost. California is now facing bankruptcy, and has been ordered by the Supreme Court to release 30,000 inmates to relieve dangerous overcrowding.
We don't have to go there. The prison population in June this year was 8708; by June 2021 it's projected to fall to 8165. We have accommodation for 9560. According to the Ministry of Justice, the fall reflects the lower crime rate and greater use of diversion by the police for low-level offences.
Roger Brooking, the author of Flying Blind, a new book on the failures of our prison system to rehabilitate or correct, believes we're seeing the result of new sentencing strategies adopted in late 2007, which have led to a 40 to 50 per cent increase in the number of home detention and community-based sentences handed out by judges.
Whatever the reasons, the projections throw serious doubt on the need for the proposed 1000-bed prison in Wiri, which the Greens say is likely to cost at least $300 million.
Finance Minister Bill English was right when he called prisons "a moral and fiscal failure". Back in May, he said plans for the new prison were part of "this Government's policy, and public pressure for tougher sentences and a safer community".
That sounds like abdication. At a time of fiscal belt-tightening, it makes no sense to pour more money into a prison that the evidence suggests we don't need, simply to appease "public pressure".
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