According to the legislative history of the Controlled Substances Act, in 1970 there was not any scientific evidence that marijuana was addictive enough to be placed in schedules I or II.

The extent to which marihuana should be controlled is a subject upon which opinions diverge widely. There are some who not only advocate its legalization but would encourage its use; at the other extreme there are some States which have established the death penalty for distribution of marihuana to minors. During the hearings, Dr. Stanley F. Yolles, who was the Director of the National Institute of Mental Health, submitted a chart of fable and fact concerning marihuana . . .[Among the facts are the following statements.]

Marihuana does not cause physical addiction, since tolerance to its effects and symptoms on sudden withdrawals does not occur. It can produce habituation (psychological dependence) . . .

We know nothing in the nature of marihuana that predisposes to heroin abuse. . .

Marihuana use has increased enormously in spite of the most severely punitive laws.

(U.S. Code Cong. & Admin. News (1970) p.4603.)