Earlier quoted context omitted.
> The team found that persistent marijuana use was linked to a decline in IQ, even after the researchers controlled for educational differences. The most persistent users — those who reported using the drug in three or more waves of the study — experienced a drop in neuropsychological functioning equivalent to about six IQ points (PNAS, 2012). "That's in the same realm as what you'd see with lead exposure," says Weis…
Given that they use IQ as a measure also goes to show how little they understand the effects at this point. So take this all with a grain of salt. The best evidence at this point is that adolescent use has some effect on long term memory formation, but even that needs much more study to be understood it. Nothing in the brain works on a simple better/worse scale.
If anything, I’d say that marijuana use tends to make the brain more doubtful, even of itself. In real life this is a mostly positive trait, but it is punished by the IQ test I think.