Youths, booze and crime

Sir Geoffrey Palmer recommends a set of anti-alcohol measures to reduce crime rates.

''We've got something like 8000 prisoners and rising, and that needs to be broken, but it takes enormous political courage to do that if you're feeling the heat of public opinion at election time.''

Sir Geoffrey, as a retired politician, was comfortable labelling prisons as ''universities for crime'' and advocating unpopular law reform.

''I would increase the price of alcohol by increasing excise tax by 50%; put up the purchase age to 20; ban advertising of alcohol on television and control advertising very carefully; regulate promotions and sponsorship; have tighter trading hours; and make it compulsory for all local authorities to have an alcohol policy,'' he said.

''Those measures would reduce the risks. Otherwise, we're going to produce more binge drinkers and addicts. I'm not a wowser but having read all the literature I am extremely worried about where we're going as a society about alcohol.''

If Palmer is right about the alcohol purchase age part, then we would expect a surge in offending by 18 and 19 year olds relative to 20-21 year olds in the period subsequent to the drop in the alcohol purchase age. Doing it up properly would require crime rates by age group over the period.

The quickly available StatsNZ figures aggregate by age group 17-19, 20-24, 25-29. If the purchase age had effects, we'd expect a jump in total sentences (which includes imprisonment, home detention, community work, and everything else) among 17-19 year olds from 2000 onwards relative to their older peers. Note that this is just a first cut: we would need to standardise by population in each age cohort to do it properly. But here's the non-standardised first cut: The left axis has the total number of sentences by age group, with the blue line showing sentences among 17-19 year olds. That series dropped from 1980 through to 1992/1993, then showed a slow rise through 2008/2009, then a sharp drop. The number of sentences provided to those aged 17-19 is lower now than at any time since 1980.

The green line shows the ratio of sentences among 17-19 year olds to those among 20-24 year olds. That series dropped from 0.94 in 1981/1982 to a minimum of 0.6 in 1992/1993, rising to 0.91 in 2006/2007, then falling to 0.55 in 2012/2013. 17-19 year olds are now receiving a smaller fraction of sentences, relative to those given to 20-24 year olds, than any time since 1980. The purple line gives a similar ratio, but with 25-29 year olds as comparison group. Again, we have a sharp drop from 1980 through the early 1990s, a slow rise from 1994 through 2002, a sharper rise from 2002 through 2006, then a rapid fall through 2012/2013. The ratio this year is, again, lower than in any prior year going back through 1980.

I can't rule out demographic changes driving changes in the aggregates. I have a harder time imagining demographic changes driving changes in the ratios. You could maybe build a case for a break in sentencing relative to 25-29 year olds starting around the time of the change in the alcohol purchase age, but the drop since 2008 has been pretty remarkable.

InfoShare provides estimates of population by age going back through 1991. Assigning 1991 data to the 1991/1992 fiscal year sentencing figures, I get the following sentencing rates by age group for 1991 to present. Again, I have a hard time seeing any kind of break at the year 2000 for 17-19 year olds. There's a big spike upwards from 2005 through 2009, followed by a sharp decline. Sentencing rates among 17-19 year olds are lower now than they've been since 1991.

I'll have to see whether I can get better age-disaggregated data in this, the season of thinking about future honours projects. It's certainly fodder for a quick diff-in-diff study. And it looks like something else was going on in crime policy in the mid 2000s that will throw in a nice confound for a student to puzzle out.