A drug addiction defense has been receiving increasing recognition as an excusing condition in the criminal law for almost two decades. In 1962, the Supreme Court first acknowledged a version of the defense in Robinson v. California. In Robinson the defendant had been charged not with an act but with addiction itself. The Supreme Court struck down the California statute criminalizing drug addiction, holding that to punish a mere status or involuntary act violated the eighth amendment cruel and unusual punishment clause. In the line of cases following Robinson some courts expanded the Robinson holding into a broad drug addiction defense, and others strictly limited it. The post-Robinson decisions considered the question of addiction as a defense to certain addiction-related actions by the defendant, including possession and use of narcotics, trafficking in narcotics, and appearing in public while intoxicated.

Each decision in the Robinson line, however, was flawed by one or both of two false assumptions: (1) that moral responsibility requires an in-dwelling agent called the will, acting freely; and (2) that moral responsibility is compatible with a wholly causal explanation of human behavior, an assumption labeled by theorists, “soft determinism.” The two assumptions lie on a philosophical spectrum running from the in-dwelling agent or “free will” theory through soft determinism to “hard” determinism (or “determinism”). As one moves along the spectrum from the in-dwelling agent theory to hard determinism, the role of free choice in human action diminishes while the role of causal control increases.

Determinism holds that for every human action there exists a complete causal explanation, or a list of the events which caused or “determined” the action. In other words, according to the theory, neither “free will” nor moral responsibility exists, because both the most enlightened and the most depraved human actions are determined by chains of causation stretching back to the beginning of time and beyond the actor’s control. To excuse illegal behavior when certain causal conditions such as incapacity, involuntariness, or unconsciousness exist becomes absurd; all human actions have causal antecedents which completely explain their occurrence.