become dependent on illicit drugs than are women. The risk of drug dependence for white Americans is approximately double that for African Americans. People between the ages of 25 and 44 are estimated to be more than three times as likely than those over 45 to abuse drugs.

Adolescents are particularly vulnerable to drug dependence since they tend to suffer the behavioral consequences of dependence at lower levels of drug use compared with adults. Young people who are already dependent on other substances—typically alcohol or tobacco —are especially prone to marijuana dependence. In a study of more than 200 patients in a residential treatment program for delinquent youth, participants were found to be dependent on an average of more than three different substances. Of those patients who had used marijuana more than six times, more than 80 percent went on to become dependent on it—a far higher rate of progression to dependence than found among the general population.21

Although parents of children who use marijuana often claim that the drug provokes rebellious behavior, the adolescents in the previous study had all displayed behavioral problems before they began abusing marijuana. Several other reports echo these observations and indicate that the more troubled a child is, the earlier he or she is likely to begin drug use, abuse, and dependence.

People with psychiatric disorders constitute another group at high risk for drug abuse. An estimated 76 percent of men and 65 percent of women classified as being drug dependent suffer from at least one additional psychiatric disorder; most frequently, that disorder is alcohol abuse. In drug-dependent women, phobias and major depression are nearly as common as alcohol abuse. Antisocial personality and its predecessor in children, conduct disorder, also figure prominently in the psychiatric diagnoses of substance abusers.

Genetic factors also appear to influence whether a person will abuse drugs, including marijuana. A study of over 8,000 male twins indicates that people inherit the tendency to enjoy marijuana's effects.22 Presumably, people who try marijuana and find it pleasant are more likely to continue using it—and thus possibly abuse it—than those who do not find it enjoyable. The results of this study and a similar survey of female twins23 indi-



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