Drugs and personality: extraversion-introversion.

Eysenck theorizes that stimulants induce introversion and that depressants induce extraversion; common sense suggests opposite expectations. Scores on the extraversion scale of the Eysenck Personality Inventory yielded statistically significant differences among carefully matched series of heavy, chronic users of cocaine, amphetamine, opiates, barbiturate/sedative-hypnotics, and a comparable series of nonusers. Cocaine users and opiates users were found to be more introverted; amphetamine users, barbiturates users, and nonusers were more extraverted . These findings did not fully support either set of hypotheses. These data also implied that, if drugs influence extraversion, they do so only by suppressing it. Data from two other measures of extraversion were consistent with this hypothesis.