I read the study thoroughly and think that the relationship between MLDA and later alcohol problems is likely spurious for the following reasons: 1) At least part of the relationship seems to be mediated by self-selective cross-state migration (i.e. budding young alcohol abusers moving to states with more lenient laws), 2) The relationship was apparently NOT mediated by earlier age of drinking onset, as would be expected if the relationship was truly causal, 3) drug addiction was affected more strongly than alcoholism for whatever reason, 4) No distinction was made between a drinking age of 18, 19, or 20, and 5) The odds ratio for the relationship between MLDA <21 and later alcohol use disorders was an anemic 1.33. In epidemiology, odds ratios and (relative risks) below 2.0 are difficult to interpret and are often due to residual or unmeasured confounding, bias, or even chance. This is especially true for retrospective and non-longitudinal studies such as this one. In other words, the results are preliminary at best.

According to NIAAA data from 1981, states with a drinking age of 21 actually had HIGHER rates of alcoholism than states with lower drinking ages.

And why does Canada (MLDA 18 or 19 depending on province) not have a higher alcoholism rate than America? Ditto for most other countries with lower drinking ages? Some may say culture, but Canada's culture is not radically different than ours.