These studies come out every few years and they generally fail to account for the fact that people who suffer from psychosis will self-medicate, and weed is the first medication of choice. (Although
my dead psychotic ex-nephew in law self-medicated with industrial-strength opiates...)
We also know that the percentage of people diagnosed with psychosis has not risen, even as weed gets more popular and stronger. If 40% of regulars developed serious mental illnesses, we would certainly notice that.
It's not empirical evidence, but my buddy who has smoked every day since the early 70s is still the most intelligent person I know.