Historians can explain these shifts much better than scientists can, since they usually have less to do with the intrinsic nature of the various molecules involved than with the powers that cultures ascribe to them and the changing needs of those cultures. Cannabis in American culture has at various times held the power to foster violence (in the 1930s) and indolence (today): same molecule, opposite effect. Promoting certain plant drugs and forbidding others may just be something cultures do as a way of defining themselves or reinforcing their cohesion.
