Cognitive ability in early adulthood as a predictor of habitual drug use during later military service and civilian life: The Vietnam Experience Study

BackgroundRecent reports have linked cognitive ability (IQ) with alcohol dependency, but the relationship with illegal drug use is not well understood.MethodsParticipants were 14,362 male US Vietnam veterans with IQ test results at entry into military service in 1965–1971 (mean age 22.58) who participated in a telephone interview in 1985–1986. A structured diagnostic telephone interview was used to ascertain habitual drug use during military service (for once a week, ≥3 months) and in civilian life (in the past 12 months, ≥once a week), combat exposure, and post-traumatic stress disorder according to established Diagnostic and Statistical Manual of Mental disorders criteria (version III).ResultsIn unadjusted analysis, men with high IQ scores were less likely to be habitual users of cannabis (OR = 0.89, 95% CI = 0.86, 0.93), cocaine (OR = 0.69, 95% CI = 0.61, 0.78), heroin (OR = 0.80, 95% CI = 0.73, 0.88), amphetamines (OR = 0.90, 95% CI = 0.83, 0.98), barbiturates (OR = 0.79, 95% CI = 0.72, 0.86) and LSD (OR = 0.91, 95% CI = 0.82, 0.99) during military service and civilian life. These associations were markedly attenuated after adjustment for socioeconomic status in early and later civilian life.ConclusionIn this cohort, socioeconomic position might lie on the pathway linking earlier IQ and later habitual drug use but might also act as a surrogate for IQ. This suggests interventions to prevent drug use could attempt to improve early life IQ and opportunities for employment.