While the authors appear to be convinced that this relationship is causal, we at Twenty-One Debunked note that there are plenty of reasons that this relationship could easily be spurious. For example:
- Several other potentially important variables were not controlled for, including ones that may not have been captured by age and period effects.
- State-to-state variation was not explored at all.
- Arrest rates may have given biased estimates due to underreporting and changes in reporting rates and police practices over time, which can yield specious inferences.
- The strengths of the underage drinking laws themselves were not explored, only the enforcement of such laws as measured by arrest rates.
- Cohorts exposed to drinking ages of 18, 19, 20, and 21 were all lumped together, with no attempt to distinguish between them.
- No other crimes were explored besides assault and vandalism, and no distinction was made between types of assault (i.e. simple vs. aggravated).
- Allocating more resources towards arresting underage drinkers (and those who commit other victimless crimes) takes away from resources used to fight real crime, including assault and vandalism. So, an increase in the former could lead to a spurious decrease in arrests for the latter in the absence of any real change in the latter.
The example of San Francisco is highly instructive in this regard. In 1990-1992, they were a crack-infested, gang-ridden hellhole. By 2000, violent crime had plummeted by half (and even more so for juveniles), and by 2009 they became one of the safest big cities in America. And how did they manage to do this? Did they employ a "broken-windows," zero-tolerance approach to the most minor offenses, especially by young people? Hardly! In fact, beginning in 1992 (the city's peak year for violent crime), they stopped enforcing their youth curfew law (which was completely abolished in 1995), and since then they have actually cut back on arresting young people for "status" offenses (such as underage drinking) and cannabis possession, making such offenses the lowest priorities. In other words, "don't sweat the small stuff." Meanwhile, the police freed up more resources to tackle serious crime, and managed to build better relations with the community. While the exact reasons for the drop in crime are not entirely clear, and several other cities nationwide saw similar improvements, it certainly casts doubt on the authors' thesis that cracking down on underage drinking reduces crime.
Interestingly, 'Frisco teens also show significantly lower rates of violence, "binge" drinking, driving after drinking, drinking in general, cannabis use, huffing, and crack/cocaine use compared with the national average according to the Youth Risk Behavior Survey. Furthermore, the rates in San Francisco are comparable to or lower than those in NYC (and dropped at a similar or faster rate) despite the latter city's notoriously heavy-handed police tactics under Mayors Giuliani and Bloomberg.