While I won't dispute that marijuana may magnify paranoid thoughts, I will say that it's just as likely that such an effect wouldn't not really matter if there wasn't something to be paranoid about. When I lived in North Carolina, Brazil and other places with laws proscribing harsh punishments and strict enforcement for possession, I regularly encountered people somewhat paranoid when high. Since living in San Francisco, I can't think of the last time I talked to someone who was acting/feeling paranoid while high.