The Journal of the International Neuropsychological Society published a review of longitudinal studies looking for a connection between cannabis use and neuropsychological functioning. The Miami-based researchers noted that such a connection has been suggested in the past, but they utilized more "informative design strategies" to determine underlying causal relationships and other information.
In summarizing the primary findings across these studies, the researchers said, "This review suggests that cannabis use leads to neuropsychological decline."
Longitudinal studies involve repeated observations of the same participants over long periods of time, and this particular study reviewed the current longitudinal data available on cannabis. This was ultimately a summary of other people's research, and it's worth noting that other clinical findings reached different conclusions. Fittingly, the researchers peppered their finding with caveats that suggest they too have doubts.
"Across most studies, these associations were modest, were present only for the group with the heaviest cannabis use, and were often attenuated (or no longer significant) after controlling for potential confounding variables," the authors continued.
The researchers found that this decline occurs under certain conditions and with shared risk factors, and future cannabis studies should seek to determine what these conditions and risk factors are. That is to say, identify the variables that contribute to the decline and the people who are most vulnerable and treat them accordingly. Obviously this approach makes a lot more sense than a national prohibition on cannabis.