For decades, prohibitionists have claimed that marijuana is a “gateway drug” that inevitably leads to use of harder substances like heroin and cocaine — despite the fact that every objective study ever done on the gateway theory has determined that it’s absolute crap. Last week, researchers at the University of New Hampshire released yet another study discrediting the gateway theory. Their findings, based on survey data from more than 1,200 students in Florida public schools, showed that a person’s…