Keeping it 18

New Zealand's reduction in the alcohol purchase age from 20 to 18 did not substantially worsen outcomes for youths. Alcohol consumption did not increase among 18 and 19 year olds subsequent to the change. Consumption, binge drinking, and accident rates among 15-17 year olds seems to have fallen since the reduction in the purchase age as well.

Stefan Boes and Steven Stillman's paper using regression discontinuity design and difference-in-difference measures on the alcohol purchase age, previewed here, is now up as an IZA working paper. Their abstract:

This paper examines the impact of a reduction in the legal drinking age in New Zealand from 20 to 18 on alcohol use, and alcohol-related hospitalisations and vehicular accidents among teenagers. We use both a difference-in-differences approach and a regression discontinuity design (RDD) to examine the impact of the law change. Our main findings are that lowering the legal drinking age did not appear to have led to, on average, an increase in alcohol consumption or binge drinking among 15-17 or 18-19 year-olds. However, there is evidence that the law change led to a significant increase in alcohol-related hospital admission rates for 18-19 year-olds, as well as for 15-17 year-olds. While these increases are large in relative magnitude, they are small in the absolute number of affected teenagers. Finally, we find no evidence for an increase in alcohol-related vehicular accidents at the time of the law change for any teenagers. In an important methodological contribution, we show that one approach commonly used to estimate the impact of changing the legal drinking age on outcomes, an RDD that compares individuals just younger than the drinking age to those just older, has the potential to give misleading results. Overall, our results support the argument that the legal drinking age can be lowered without leading to large increases in detrimental outcomes for youth.

The difference-in-difference measures in particular show lowered consumption among 15-17 year olds.

Finer-grained data on traffic accidents showed no increase with the purchase age reduction. There were more youth hospital admissions for alcohol use disorders subsequent to the law change. But while the increase is large in percentage terms, the baseline risk is low. Boes and Stillman estimate an additional 2.2 to 3.4 alcohol use disorder hospital admissions per month among 15-17 year olds, 2.1 to 2.6 per month among 18-19 year olds, and 1.0 to 1.5 per month among 20-21 year olds in the period immediately following the law change. They note that seasonal variation in admission for flu and asthma are three and twenty times larger than the variation attributed to the reduction in the alcohol purchase age.

They also make an important point on method, and I thank Steven for having explained this one to me slowly; hopefully I've understood it properly. Now recall that they found no effect of the law change on traffic accidents: changing the alcohol purchase age from 20 to 18 did not increase the accident rate among youths. Nevertheless, results from a regression discontinuity design comparing accident rates among kids just under the alcohol purchase age with those just over the alcohol purchase age shows an increase in accidents on reaching the alcohol purchase age after the law change. Why could there be an effect in RDD but not overall? Either the RDD is picking up the effect of the alcohol purchase age on the cohort of inexperienced drinkers who change their behaviour on reaching 18 and experience worsened outcomes, or it's picking up a particular changed behaviour around the time of the birthday. We would have overestimated the effects on traffic accidents for 18-19 year olds by extrapolating from the discontinuity around the 18th birthday.

But, we only know this because New Zealand actually had the policy change so we can compare actual outcomes with those estimated by the RDD. Now, suppose you're in a country that hasn't reduced its drinking age and you want to estimate what the effect of decreasing the drinking age might be. If you ran an RDD looking at the effect of reaching the age of majority on traffic accidents, took the break around the birthday as being the effect of being able to drink, then extrapolated that effect back across the cohort of younger drinkers who would be eligible to drink under a different minimum legal drinking age, you could pretty easily be overestimating the likely effect of a real change in the alcohol purchase age. The RDD picks up the effect of reaching the legal drinking age rather than the effect that would obtain by lowering the legal drinking age.

They conclude:

Overall, our results support the argument being made by groups like Amethyst Initiative and Choose Responsibility (see http://www.choose responsibility.org/proposal/) that the legal drinking age can be lowered without leading to large increases in 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.

It is pretty tough to maintain a position that youth drinking is any kind of worsening problem in New Zealand.

Earlier this month, Auckland University's Youth Survey came out. Binge drinking rates among high school students have almost halved since 2001. The NZ Herald highlighted a few other trends from the survey hardly consistent with any kind of growing youth crisis. David Farrar linked up the current survey numbers with prior year figures to show decreases in all measures of youth alcohol consumption.

Nutty about the alcohol purchase age

CloseUp tonight (24 October) featured David Nutt on New Zealand's alcohol reforms. In the clip below, he says that New Zealand's lowering of the drinking age [actually, the alcohol purchase age] provides example of the increased harms and increased traffic accidents that eventuate.

Here is the best evidence we have on the effect of lowering the alcohol purchase age on traffic accidents.

It is hard to see evidence of substantial harm consequent to New Zealand's lowering of the alcohol purchase age from 20 to 18.

Difference in Difference and the Alcohol Purchase Age: A Stillman Preview

Reducing the alcohol purchase age to 18 really doesn't seem to have increased youth drinking, or at least not on the aggregate statistics. I couldn't see any case for worsened outcomes on the aggregate trends. Steve Stillman kindly forwards the tables from a paper he's now finalising using proper difference-in-difference and regression discontinuity design techniques to look at the effect of the alcohol purchase age change. 

Difference-in-difference method compares changes in a control group's behaviour with those in the treatment group. The counterfactual then is that changes in drinking among youths would have mirrored changes in drinking among 22-23 year olds but for that the alcohol purchase age dropped to 18.

In Table 3, below, Stillman reports difference-in-difference results on drinking behaviours. And I'll apologise here for pasting in an image of Stillman's tables; when he has the full paper up, I'll link it.

In the models with statistical controls for income, ethnicity, deprivation index and the like, Stillman finds evidence of relative reductions in drinking among younger cohorts. Fifteen to seventeen year olds reduce their binge drinking rates, relative to the change among 22-23 year olds, and also reduce their likelihood of suffering alcohol-related injury.  By 2006/2007, that cohort also was consuming fewer drinks per drinking session and had reductions in the log of total consumption, again relative to consumption among 22-23 year olds. 

Looking at Part B of the table, which reports the results from specifications controlling for other covariates, we find some evidence of improved outcomes among 15-17 year olds and no evidence of worsened outcomes. 

The binge drinking rate among 15-17 year olds was 13.3% in 1996/1997. Steve kindly, and patiently, corrected my initial misreading of the results: the -0.15 coefficient suggests a 15 percentage point reduction from 13.3 rather than a 15% reduction. I had incorrectly assumed that the tables presented transformed results as the fifteen percentage point reduction seemed implausibly large - binge drinking rates obviously cannot be negative. The results then tell us that binge drinking must have increased among the 22-23 year old control group. 

Similarly, the rate of alcohol-related injury dropped substantially relative to that experienced among 22-23 year olds. 12.7% of kids in that age cohort reported ever having had an injury subsequent to drinking in 1996/1997. The 8.8 percentage point reduction in the injury report rate would seem to have dropped reported injuries to a third of their rate prior to the change in the purchase age but, again, the counterfactual here is that the injury rate among 15-17 year olds would have increased in line with the increase among 22-23 year olds. 

Relative to the counterfactual that 15-17 year olds' results would have mirrored those among 22-23 year olds, the difference-in-difference estimates tell us that 

more than 7,300 fewer kids presented with alcohol related injuries than we would have otherwise expected and that more than 12,000 fewer kids participated in binge drinking.

The model cannot distinguish between changes caused by the reduction in the purchase age and anything else that might have simultaneously affected binge drinking rates among kids but not among the comparison cohort, or that might have caused young adults in the 22-23 year old reference group to start binge drinking. But it’s hard to point to other policies that would have had the age-specific effects.

Results on reductions in drinks-per-session are also substantial: a one drink reduction where the mean otherwise was just over three. The rest of the table shows no significant effects of the reduction in the alcohol purchase age: no increase in the likelihood of drinking or in the frequency of drinking. I couldn't see anything in the aggregate stats suggesting increases in youth drinking after the reduction in the purchase age; Stillman can't find it using difference-in-difference either.

Doug Sellman had suggested that binge drinking rates could mask what's going on in the upper tail of that distribution: those who binge drink might be consuming a lot more harmfully consequent to the change. I suppose we can't rule it out, but per-session consumption and the log of total consumption are both down; the maths then require that group is fairly small or that everyone else's consumption is down by more than is suggested above.

Stillman finds evidence of increased hospital admissions for alcohol use disorder. But, the effects are not large. Figure 1, below, uses regression discontinuity design (RDD) with a break at the time of the law change. RDD isn't possible for binge drinking because we there only have three waves of annual data. The advantage of RDD over difference-in-difference is that we can look at the period right around the change in the purchase age, making it unlikely that other changes have affected outcomes. But, if it takes a while for behaviours to change subsequent to policy changes, RDD will have a harder time picking that up.

Yup, there's a break at the time of the law change. The biggest effect was among 18-19 year olds. Alcohol-related hospital admissions rose from about 6 per 100,000 to about 10 per 100,000 per month. There were about 55,000 kids in that age cohort at the time of the law change. So the RDD design suggests that the law change resulted in perhaps two additional 18-19 year olds per month showing up for treatment. It's a relatively large proportional change, so regressions will show large percentage effects. But again, the base rate is tiny. In difference-in-difference estimation, the largest estimated effect was a 49.6% increase in hospital admissions related to an alcohol use disorder among 18-19 year olds three years after the law change. That corresponds to about three people per year.

What about traffic accidents? Stillman finds a small initial decrease in alcohol-related car accidents among 18-19 year olds after the purchase age declined, followed by an increase. 

The regression discontinuity design finds no significant effect. When Stillman runs the difference-in-difference estimates, he finds no effect of the change in alcohol purchase age on alcohol-related traffic accidents in models with a full range of demographic controls that allowed for age-specific time trends. A model with a more limited set of control variables suggested no effects on traffic accidents until three years after the law change. At that point, accident rates increased by 13.7% among 15-17 year olds and by 25.3% among 18-19 year olds. While the percentage increase sounds large, again, the base rate is tiny: the estimates correspond to an increase from 0.5 to 0.57 car accidents per month for 15-17 year olds, or an increase of about one car accident per year. For 18 and 19 year olds, we went from 1.2 accidents per month before the change to 1.46 afterwards: 3.5 additional accidents per year. And, again, those effects disappear with the addition of age-specific time trends, which might just matter once we get to three or more years from the time of the policy change.

In short, it’s pretty hard to build a case for substantially worsened outcomes consequent to the drop in the alcohol purchase age; doing so requires relying on estimates without the full set of desired controls and putting a lot of weight on a pretty small absolute change in the number of accidents. Further, since negative effects on accidents really only seemed evident from three years subsequent to the alcohol purchase age, we have to worry whether anything else happened that differentially affected youth versus adult drink driving rates. And, the results from difference-in-difference estimation suggested pretty substantial reductions in youth binge drinking subsequent to the reduction in the alcohol purchase age, relative to the older comparison cohort.

We are very lucky that clever herestheticians prevented the drinking age from rising. It would have imposed substantial costs on 18-20 year olds for no real benefit.