[beat]

You'd have to google around for authoritative sources, but the history is pretty straightforward. Opium/heroin prohibition started as anti-Chinese policy. Marijuana prohibition was drenched in anti-black and anti-Mexican racism, as using it for intoxication was virtually unknown in the white community (although it was often an ingredient in "patent medicines").

There's a healthy bit of conspiracy theory that says the real motivation for the marijuana prohibition was a Dupont family political move against Henry Ford, who was looking at alternative fuels based on hemp. Certainly, Harry Anslinger, the primary campaigner for anti-marijuana laws, was a Dupont in-law, lending credence to the theory. Still, it's conspiracy theory and should be taken with a grain of salt.

But in the longer political run, marijuana makes an excellent nuisance crime for harassing social undesirables - blacks, latinos, and white hippies and artists who were unduly influenced by black culture. That's not a matter of research, it's just plain obvious. And since it's kind of hard to actually show that marijuana causes black men to rape white women or that it leads directly to heroin and communism or whatever, there is quite a cottage industry in faux-science to "prove" the dangers of a basically harmless herb.

The bigger problem in modern years, imho, has been the rise of the private drug testing industry. It doesn't hurt the self-image of big businesses to demand that potential employees pee in a cup to prove their competence, but it's completely shameful socially. It's ridiculous.