The last time Northwestern did a study on the effects of cannabis on the brain [Gilman et al. 2014] they claimed something similar but had horrible controls, the cannabis using group used much more alcohol and cigarettes than controls. That study was soon refuted with a new study that properly controlled for alcohol use [Weiland et al. 2015].

The authors of this new Northwestern study admit causation has not been determined: "Although the results may be compatible with a causal hypothesis, we must consider that the observed cannabis-related shape differences in the hippocampus could also be explained as biomarkers of a neurobiological susceptibility to poor memory or the effects of cannabis". There were only 10 subjects in this study who used cannabis heavily as teens, this is not a large sample.

Regardless, only a legal, regulated system can reduce regular teen access. It is better to have some control over the 20 millions pounds traded yearly in the U.S. rather than no control.