The controversy over the cause-effect relationship between marihuana use and criminal, violent or delinquent behavior poses a number of serious problems for the investigators Proponents and opponents of the causal view tend to rely on different kinds of evidence and to call upon different types of experts, thereby differing substantially in the kinds of information they accept as relevant, reliable or valid.

Practitioners, such as police and probation officers for example, frequently cite case examples in which apprehended offenders are found to be in possession of marihuana at the time of arrest. The mere presence of the drug or the fact that an offender is a known user of marihuana is sometimes deemed sufficient to establish a causal link between the marihuana and the offense.*

Empiricists, on the other hand, would deny that the simple presence of the drug constitutes a satisfactory demonstration of a causal relationship between marihuana use and the crime in question. They would defer, instead, to the results of empirical studies designed explicitly to test the assertion. Essentially, they emphasize that even if some offenders do use marihuana, an equal or larger number of offenders do not, and there are certainly large numbers of marihuana users in the population-at-large who never engage in the kinds of antisocial conduct deemed to be related to or caused by the use of the drug.

Proving any positive and direct relationship, be it causal or otherwise, between two inherently complex social phenomena is fraught with enormous difficulties. The relationship of marihuana use to crime, violence, aggression or juvenile delinquency presents no exception. Before examining the evidence with respect to the existence of a causal connection, certain basic considerations deserve at least brief mention here.

To prove, the existence of a positive and direct relationship, one would be required to demonstrate that the alleged offender was, indeed, a marihuana, user; that he was under the influence of the drug at the time he committed the offense; and that the crime was directly attributable to the effects of the marihuana. The kinds of evidence necessary to establish these facts are not easy to obtain.

First, evidence of the use of marihuana by the accused is generally dependent upon either direct admission of use, hearsay evidence, or inferences derived from knowledge of possession (that is, the offender was found to have marihuana on his person or in his possession at the time of arrest).

Second, because no chemical tests presently exist outside the laboratory to identify the presence of marihuana in the body of the accused, it is difficult if not impossible to prove that the offender was definitely under the influence of marihuana when he committed the offense.

Third, in order to prove that the marihuana represented the significant contributory or precipitating variable, all other factors possibly related to the offense would have to be examined and excluded.

The problems of validation are further compounded by additional variations in behavior attributable to: (a) the pharmacological potency of the drug; (b) possible adulteration of the marihuana; (c) the interaction of marihuana with other drugs simultaneously ingested; (d) differing individual response to similar dosage levels; (e) the time-action function; (f) the cumulative effect of marihuana use; and (g) various social, psychological and situational variables such as set and setting, individual expectations, personal predispositions or preexisting impulse disorders.

Despite the inherent complexities of the issue and the difficulties in securing reliable and valid evidence, a relatively large body of research is now available pertaining to the criminogenic effects of marihuana upon the individual and the nature and extent to which the drug constitutes a danger to public safety. In the following section, we present the available evidence and assess the strength and direction of the alleged relationships between marihuana use and violent or aggressive behavior and also non-violent forms of criminal and delinquent behavior.

*In the widely publicized Licata case of the 1930's. for example, a 16-year-old cannabis user was charged with the ax murder of his family and the offense was directly attributed to the effects of marihuana. There was, however. no precise information available regarding the use of marihuana in relation to the crime. Nor. in the various accounts of the case, was there generally any reference to the fact that several of the boy's relatives had previously been committed to mental institutions; that the police hall, about one year prior to the offense (and presumably before the youth's alleged use of marihuana) attempted to commit him for his bizarre behavior; or that shortly after the crime, the boy began to exhibit the symptoms of paranoid schizophrenia.