A recent study has shown that there’s little to no effect on intelligence at any age with smoking marijuana. The study was conducted by reviewing data collected between twins, where one twin used marijuana over an extended period of time and the other did not.
Throughout the debate prohibitionists have not been able to point to proof that directly shows cannabis use having a detrimental effect to a young person’s intelligence. The study itself reads a little dry, but is interesting. Here’s a sample:
Among pairs of twin discordant for marijuana use, we assessed intelligence quotient (IQ) score changes while adjusting for the effects of genetic influences and other factors shared by members of the same twin pair. Results suggest that familial confounds underlie the association between adolescent marijuana use and declining IQ scores