Researchers followed 789 pairs of twins, beginning at the age of 9–11 and, over the next decade, administered five intelligence tests and asked participants about their use of cannabis, along with other narcotics such as alcohol, opioids and cocaine.
While those participants who admitted to using cannabis lost around four IQ points over the investigation period, their twins, who reported no cannabis use, declined by the same amount – a result that suggested environmental factors other than marijuana that both twins shared, such as home or school life, to blame for the change.
“Observed declines in measured IQ may not be a direct result of marijuana exposure but rather attributable to familial factors that underlie both marijuana initiation and low intellectual attainment,” the report read.
Study researcher Joshua Isen said the results are likely to have little impact on legalization efforts as participants were under the age of 18 when they began their cannabis use.
“Even if cannabis is legalized, there will still be a minimum age requirement (much like for cigarettes and alcohol),” Isen wrote in an email. “Recent research suggests that cigarette use and binge drinking are even more negatively associated with intellectual functioning in adolescents. If legislators are willing to allow these other substances (nicotine and alcohol) to remain legal, then there is really no case for outlawing adult consumption of cannabis.”