Marijuana use during adolescence has been associated with reduced intelligence, memory, attention, and verbal ability. However, whether marijuana use causes reduced cognitive ability or if the association results from confounding factors remains unclear.

Researchers examined the relationship between marijuana use and various standard measures of intelligence in two longitudinal twin studies comprising more than 3,000 individuals.

Study participants who had used marijuana showed significantly greater decreases in measures of crystallized intelligence between preadolescence (ages 9–12) and late adolescence (ages 17–20) than nonusers. However, among users there was no association between the level of marijuana use and the decline in intelligence.

To control for potentially confounding genetic and familial factors, the authors compared intelligence scores within twin pairs in which only one twin used marijuana. No significant differences in intelligence were observed between the marijuana-using twin and the nonusing twin in both fraternal and identical twin pairs.

The results suggest that the cognitive decline observed in marijuana users may not be directly caused by marijuana use. Instead, both marijuana use and cognitive ability might be influenced by aspects of the family environment, according to the authors. 

http://www.pnas.org/content/early/2016/01/13/1516648113