But marijuana acts differently on the mind and for much longer periods.....and destroys incentive. I know brilliant older men who drink beer every day and work hard, whose brain remains sharp. Not so with marijuana—they become really goofy and lazy. Water was not that safe back in colonial days.
Firstly, because marijuana is illegal its users don't advertise their use, so the only ones whose use becomes known to you are the goofy and lazy ones.
Secondly, it's at least as likely that any cause and effect relationship runs the opposite way: that people who are already goofy and lazy are likelier than average to turn to marijuana use.