Wonderful. Another study with no basis for distinguishing causality from correlation.
I bet you can randomly select a subgroup of 18-percent poorer performers and find that they have all sorts of things in common, from diet to exercise to presence or absence of parents - as well as bizarre factors like who was born on a Tuesday.
There are probably dozens of markers that both correlate with these kinds of results and also plausibly suggest an explanation.
> “It is possible that the abnormal brain structures reveal a pre-existing vulnerability to marijuana abuse,” Smith said. “But evidence that the longer the participants were abusing marijuana, the greater the differences in hippocampus shape suggests marijuana may be the cause.”
What?! No it doesn't. It suggests quite the opposite: that people with greater abnormality tend to use marijuana longer.
I suspect that, now that medical cannabis is widely available and somewhat widely used in the treatment of childhood maladies, we may begin to see, in the next decade or so, results based on much sounder science simply because of the availability of higher quality sample groups.