In the present study, we investigated the association between persistent cannabis use—prospectively assessed over 20yrs and neuropsychological functioning in a birth cohort of 1,037 individuals. Study members underwent neuropsychological testing in 1985 and 1986 before the onset of cannabis use and again in 2010–2012, after some had developed a persistent pattern of cannabis use.

First, we tested the “cognitive decline” hypothesis that persistent cannabis users evidence greater decline in test performance from childhood to adulthood than nonusers. By examining within-person change in neuropsychological functioning, any effect of premorbid deficits on later (postcannabis-initiation) test performance was nullified.

Second, we tested the “specificity” hypothesis to address whether impairment is confined to specific neuropsychological domains or whether it is more global. To test this hypothesis, we administered multiple tests for each of five specific domains, as different tests may be differentially sensitive to cannabis-associated neuropsychological impairment. In conducting our analyses, we tested alternative explanations for the association between sistent cannabis use and neuropsychological functioning by ruling out potential confounding effects of (i) acute or residual cannabis intoxication, (ii) tobacco dependence, (iii) hard-drug dependence (e.g., heroin, cocaine, amphetamines), (iv) alcohol dependence, and (v) schizophrenia.

Third, we tested the “education” hypothesis that persistent cannabis users experience neuropsychological decline simply because they have eschewed academics and other opportunities for learning. Recent evidence suggests that staying in school can boost one’s intelligence quotient (IQ) (22), and cannabis users tend to receive less schooling than nonusers (23). Therefore, we tested whether the association between persistent cannabis use and neuropsychological decline remained after controlling for years of education.

Fourth, we queried third-party informants to test the “everyday cognition” hypothesis that cannabis-induced neuropsychological impairment translates into functional problems in daily life.

Fifth, we tested the “developmental vulnerability” hypothesis that individuals
who begin cannabis use as adolescents are particularly vulnerable to the effects of persistent cannabis use on neuropsychological functioning, as evidence suggests that cannabis has especially toxic effects on the developing brain (24–31).

Sixth, we tested the “recovery” hypothesis that former persistent users who quit or reduce their cannabis use may be able to restore their europsychological health.