On 1 December 1999, New Zealand lowered the minimum legal purchase age (MLPA) for alcohol from 20 years to 18 years. That set in place an interesting natural experiment for how legal access to alcohol affects young people's behaviour. I've blogged before about research showing the effects on hospitalisations (where there appears to have been an increase, but it's unclear how large or otherwise the increase was) and alcohol-involved motor vehicle crashes (where the evidence is weak). What about the effects on youth crime in New Zealand? After all, the research I referred to in this post a couple of months ago seemed to demonstrate about a 15.7 percent increase in crime at the age where alcohol becomes legally available (age 16 in Germany).
In a recent article published in the journal Oxford Bulletin of Economics and Statistics (open access), Kabir Dasgupta, Alexander Plum, and Christopher Erwin (all Auckland University of Technology) looked at how youth crime changed when the MLPA decreased (see also their non-technical summary in The Conversation). They use administrative data from Statistics New Zealand's Integrated Data Infrastructure, and looked at monthly periods for young people in 1994-1998 (when the MLPA was 20 years) and 2014-2018 (when the MLPA was 18 years). Specifically, they look at what happens to crime rates before and after age 20 for both groups, using a regression discontinuity design. They also employ a difference-in-differences approach, comparing youth aged 18-19 years with youth aged 20-22 years, before and after the change in MLPA.
Their first method (regression discontinuity) is probably best summarised by Figure 1 from the paper. First, the results for when the MLPA was 20 years (in 1994-1998):
Notice that there is a big decrease in court charges (their measure of crime) at age 20, but that drop is entirely driven by a decrease in age-dependent traffic offenses (which are mostly violations of drink-driving laws, where the breath alcohol limit differs for drivers younger than age 20). Indeed, their regression analysis shows that:
Overall, focusing on the more comparable age-independent crime indicators, we do not find any significant change in alcohol-related crimes when the MLPA was 20.
As for the more recent sample, when the MLPA was 18 years (in 2014-2018):
The pattern looks fairly similar, with the same drop in crime at age 20, driven by a large decrease in age-dependent traffic offenses. At first glance, there doesn't appear to be an up-tick in crime at age 18 (which is the MLPA in this sample). However, based on the regression analysis Dasgupta note that:
At the relevant MLPA threshold, we do not find any statistically discernible change either in the overall measure or in the age-independent measure of alcohol-related crime...
Looking at alcohol-induced traffic convictions, we do not observe any significant effect for the age-independent measure, which excludes BBAC limit violations. There is however a statistically significant (at the 1% level) jump in age-dependent traffic convictions at the 18-year age threshold. Specifically, an increase of approximately 7.5 convictions per 100,000 population ... indicates that gaining alcohol purchasing rights triggers a rise in infractions of mandated BBAC limits applicable to youth below 20. Compared with the sample mean of groups just under the relevant MLPA, the linear RD coefficient represents a 27 % (7.5×100/27.4) increase in alcohol-induced age-dependent traffic convictions.
So, there is a small increase in traffic violations at age 18 in this sample, which is likely to be mostly an increase in age-related drink driving offences. In further analysis, Dasgupta et al. show that:
...there is a significant increase in age-dependent traffic convictions among individuals residing in non-urban (rural) locations and those living in socio-economically more deprived neighbourhoods.
So, the results are concentrated among youth who live in poorer and more rural locations. However, before we get too carried away, we need to recognise that Dasgupta et al. are comparing a sample from 1994-1998 with a sample from 2014-2018. That wouldn't be my first choice of comparison group, not least because youth drinking norms have been changing over time (see here and here). It would be better to compare groups that are closer in time, and that's what Dasgupta et al. do in their difference-in-differences analysis (which they only present as a bit of a robustness check late in the paper). In that analysis, they report that:
...we do not find any significant evidence of an increase in criminal activities in the post-1999 period for young individuals who gained purchasing rights for the first time.
Dasgupta et al. don't actually show the results of that analysis in the paper, which is a bit disappointing, as I would have found it more compelling than the results they actually did report. Overall, they conclude that their results show:
...little evidence that late adolescents commit more alcohol-related crimes upon crossing over the legal purchasing age in New Zealand.
That's not quite true for when the drinking age is 18, but if most of the increase in crime at age 18 is drink driving violations that relate to a lower breath alcohol limit for those aged under 18, it isn't clear that there's a strong case for an increase in harm. However, although statistically insignificant, there is an apparent increase in crime overall at age 18 in the regression discontinuity results. It is relatively small, at 4 convictions per 100,000 population, or a 7.6 percent increase. That's not insubstantial. I'd be much more cautious about claiming that there is 'little evidence' of an increase in crime. With the prevalence of youth drinking way down, it's not at all clear how much statistical power an analysis that looks at all young people has. It would be interesting to see what the results would be, if limited only to youth who drink.
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