Fergusson's team looked at a group of 1,265 New Zealand kids who were followed from birth to age 25 and assessed at various points along the way for a variety of physical, mental and social problems and issues. At ages 18, 21 and 25 they were assessed for both marijuana use and supposed psychotic symptoms. Having found a correlation, with daily users reporting the highest frequency of psychotic symptoms, they then applied a series of mathematical models. These models are designed to adjust for possible variables that might confound the results and to assess whether the marijuana use caused the symptoms or vice versa.
Whatever model was applied, the correlation held up. But the reported "growing evidence" that "regular use of cannabis may increase risks of psychosis" depends completely on the validity of the underlying data, and those data raise some screamingly obvious questions.
Psychotic symptoms were measured using 10 items from something called Symptom Checklist 90. Participants were asked if they had certain ideas, feelings or beliefs that commonly accompany psychotic states. The researchers did not look at actual diagnoses, and the symptom checklist is not identical to the formal diagnostic criteria listed in the DSM-IV manual. Perhaps most important, they only used 10 "representative" items from a much larger questionnaire.
These 10 items focus heavily on paranoid thoughts or feelings, such as "feeling other people cannot be trusted," "feeling you are being watched or talked about by others," "having ideas or beliefs that others do not share." This presents a big methodological problem, because it is well known that paranoid feelings are a fairly common effect of being high on marijuana.
But the article gives no indication that respondents were asked to distinguish between feelings experienced while high and feelings experienced at other times. Thus, we are left with no indication at all as to whether these supposed psychotic symptoms are long-term effects or simply the normal, passing effects of marijuana intoxication.
Text