One potential criticism of the present analyses is our reliance on retrospective reports. Although prospective data would be preferable in many ways, the present data allow us to assess longer-term outcomes for events whose period of risk may extend well beyond substance initiation. To address potential concerns about our reliance on retrospective recall for age of initiation, we reran our analyses defining pairs as discordant only if they reported ages of initiation that differed by at least 2 years, and found no evidence that the pairs discordant by a short interval were driving the results observed in the original analyses (results available from the first author upon request). In addition, the present results are based on a sample of male Vietnam-era veterans who were predominantly Caucasian, and may not generalize to other ethnicities, to women, to non-veterans, or to cohorts who grew up in an era with different levels of acceptance of and/or access to cannabis.
