Jacob Sullum reviews Smoke Signals: A Social History of Marijuana—Medical, Recreational, and Scientific by Martin Lee. How racism and propaganda led to the banning of the plant:

Marijuana’s association with blacks and Mexicans, which marked it as an exotic drug used by inferior but scary outsiders, proved crucial to its prohibition. The bans began at the state level in 1915, when California outlawed the plant, and culminated in the federal Marihuana Tax Act of 1937. With marijuana as with opium, Lee observes, "the target of the prohibition was not the drug so much as those associated with its use."

Federal Bureau of Narcotics Commissioner Harry Anslinger warned that "marihuana causes white women to seek sexual relations with Negroes" and claimed that half the violent crimes in areas occupied by "Mexicans, Greeks, Turks, Filipinos, Spaniards, Latin Americans, and Negroes may be traced to the use of marihuana." Anslinger, who collected and circulated accounts of bloody crimes allegedly caused by marijuana, portrayed it as "the most violence-causing drug in the history of mankind" (a title that has since been seized by a succession of other drugs, often based on equally dubious evidence).