Regarding "Pot on the Spot: Marijuana's risks become blurrier" (SN: 5/22/04,
p. 323: Pot on the Spot: Marijuana's risks become blurrier), it seems to me that the stronger the social pressure against using marijuana in a culture, the more likely it will be that those who use it will be troubled, antisocial, or unusually independent. Hence, any increase in the incidence of mental problems after using marijuana could be attributed to an increased likelihood that those prone to having mental problems would be among those who use it.
People's brains aren't fully developed until they're in their twenties. Pot is well documented as an antimotivational drug used for escapist fantasy. Teens need to first operate in the real world and let their brains develop to deal with reality. The same argument goes for alcohol.