The RAND Corporation's Drug Policy Research Center released a study Monday that casts grave doubt on the validity of the "gateway theory," the intuitive but unproven notion that the use of marijuana leads to the use of harder drugs. The "gateway theory" has guided US drug policy for a half-century and has been used by prohibitionists to justify imposing tough penalties for even the possession of small amounts of marijuana. In recent months, drug czar John Walters and others of his breed have seized on the "gateway theory" to campaign against relaxing marijuana laws in the states. While the RAND...