alcohol consumption and state beer taxes in the year that the respondent turned 18. However, a possible concern in a study design of this kind is that some other, unobserved social process might better explain an apparent link between MLDA policies and adult substance use patterns. In the case of MLDA laws, it is possible to further narrow the field of competing explanations by comparing MLDA effects in birth cohorts that fall naturally in to two contrasting periods. In the earlier wave, born 1948–1955, legal purchase ages were being lowered along with the age of majority for many other legal rights; younger respondents in this era were exposed to a more permissive political and drinking environment than older respondents living in the same state. In the later wave, born 1956–1970, legal purchase ages were rising because of specific public concerns about youth drinking, and younger subjects were exposed to a less permissive drinking environment. If higher purchase ages have similar effects in both earlier and later cohorts, then the association cannot be explained by age at assessment or by trends that moved in the same direction across both periods. In another set of analyses, we limit the sample to subjects
