Exactly. Did they carefully check for family history of psychosis? Most of these studies do not look carefully at family history and the ones that do find that

Exactly. Did they carefully check for family history of psychosis? Most of these studies do not look carefully at family history and the ones that do find that heavy cannabis users have more family members with psychosis, fully explaining any link between cannabis and psychosis. Similarly, people who study math have a greatly increased rate of psychosis, but nobody warns kids against studying math because the math-psychosis link is fully explained by family history/genetics. Before people become overtly psychotic there is a long "prodomal" period where they are just a bit weird, in this prodomal period period pre-psychotic people can have an interest in math and cannabis.