The NYS sample is drawn from seven cohorts, aged 11-17 in the first year of data collection (1976) and aged 18-24 in the most recent year for which data are currently available. The data are examined in the context of two hypotheses: the Maturational Reform hypothesis and the Easterlin hypothesis. The Easterlin hypothesis asserts that the size of a cohort tends to be inversely related to the relative income of that cohort. The greater scarcity of resources for such cohorts in turn leads to higher levels of psychological and emotional stress. The Maturational Reform hypothesis holds that illegal behavior increases in early adolescence, is highest in middle to late adolescence, and then declines in early adulthood. For the prevalence of general delinquency, the study found, in descending order of magnitude, period, age (absolute deviation from age 16), and cohort size effects, confirming both the Easterlin and Maturational Reform hypothesis. Substance use appears to be subject to a Maturational Reform effect and appears to peak at the end of adolescence and the beginning of adulthood. It is affected by cohort size, as predicted by Easterlin. The prevalence of mental health problems, but not mean scale score, is consistent with the Easterlin cohort-size hypothesis, and it has a linear negative relationship to age. 11 tables.