Marijuana may not lower your IQ [while possibly enhancing your EQ?]
In Jackson and Iacono’s analysis, marijuana use and IQ were completely uncorrelated, and IQ measures fell equally in both the users and abstainers. Subsequent twin studies, including one performed with U.K. data by the Dunedin team, corroborated these findings of no relationship between marijuana use and a falling IQ.
Next, investigators uncovered shared underlying vulnerability factors that explained both marijuana use and IQ decreases. For example, behavioral traits such as impulsivity and excessive risk-taking predicted both substance use and lower IQ, as did being raised in a family that did not value education. Delinquent kids received lower grades because of their tendency to skip school and use drugs. So cannabis use was not a culprit in cognitive decline. A welter of inherited and environmental factors seemed to explain both … Because twins grow up with the same family backgrounds and are genetically very similar (nearly precisely so in identical twins), they form perfect experimental controls for each other. If Twin A smokes cannabis while Twin B doesn’t, then researchers have a tightly controlled mini experiment that helps rule out confounding factors such as Dad’s job or the alcoholism in Mom’s family. With epidemiological twin studies, a researcher is able to look across an entire sample and summarize all the relevant effects.