Reducing the alcohol purchase age

Reducing the alcohol purchase age from 20 to 18 did not increase the number of road accidents. If anything, risky driving dropped for a while.
Stefan Boes and Steve Stillman have updated their earlier work on New Zealand’s alcohol purchase age to bring in some more recent accident data to allow for longer term trends.
They make an important methodological point – one that applies to a lot of work on minimum legal drinking ages. It is really easy to set up a regression discontinuity design using the birthday as the point of the discontinuity. The 17 year old, before the birthday, is the same person a week later, except with access to alcohol. Sure. But the RDD method can’t tell you whether you’ve identified a birthday effect that will wash out over time, or a persistent negative effect resulting from a change in access to alcohol.
New Zealand makes for a good case study because the change in the minimum legal alcohol purchase age is relatively recent. So we can check what happens around birthdays before and after the change, set appropriate counterfactual trends, and look at overall effects on accident rates for those in the age group that switches status.
They conclude:

Overall, we find no evidence that changing the drinking age from 20 to 18 led to more vehicular accidents or alcohol-related accidents among teens. This is true both in the short-run following the law change and when examining cumulative accidents for the affected cohorts. We find that accidents do increase after one’s 18th birthday, but this appears to be a short-run phenomenon. Finally, our parametric regression models suggest that reducing the drinking age led to a decline in risky driving by youth who were already 15 at the time of the law change but had no longer-run impacts on youth risky driving among younger cohorts. We speculate that this occurred because of the extensive public discussion about the drinking age change that took place and because teens are likely to be particularly focused on the near future. We also present supportive evidence from infrequent health surveys showing a similar pattern for alcohol consumption among different youth cohorts. Our results support the argument that the legal drinking age can be lowered without leading to increases in detrimental outcomes for youth and call into question previous studies that have made policy recommendations by extrapolating from results identified using age-based RDDs.
Overall, our results support the argument being made by groups like Amethyst Initiative and Choose Responsibility (see that the legal drinking age can be lowered without leading to detrimental outcomes for youth. The current age limit of 21 in the US is higher than in Canada, Mexico and most western European countries. The arguments against lowering the drinking age typically include the idea that, even if a new steady-state with a lower drinking age might be beneficial, the transition to that new steady-state might be very costly. The evidence in our paper from a country with drinking habits very similar to the US suggests that this does not have to be the case.

As noted in a 2012 Stuff article:

Police have been stunned but delighted by the drop in convictions for drunken driving by teenagers – almost a year after the zero alcohol limit was brought in for the under-20s.

Figures released by police under the Official Information Act show that in first nine months of the new law coming into force on August 7 last year, 3091 youths aged 15-19 were convicted for drink driving.

The figure for the 12 months before the law change was 6414 – tracking towards an ‘‘absolutely brilliant’’ change, acting national manager road policing Superintendent Rob Morgan said.

Police had expected the introduction of the law to lead to more offences overall for young drivers, but it appeared as if the law change was acting as a deterrent  ‘‘… certainly these results are very encouraging,’’ he said.

Thanks to Eric Crampton, and to Stuff for this information.

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