Studies showing that these early temperament markers predict later substance use further support the salience of early behavioral inhibition for the internalizing pathway to SUDs. For example, Ensminger, Juon and Fothergill (2002) found that first grade boys (though not girls) who were both shy and aggressive had higher risk for drug use in adulthood. Similarly, Caspi et al. (1996) found that inhibited (fearful, shy, and easily upset) three-year-olds, compared to their peers, had higher rates of depression and, for boys, alcohol-related problems at age 21. Other studies also demonstrate that indices of internalizing behavior between ages 3 and 10 are predictive of more alcohol-related problems and disorder in mid-adolescence to early adulthood (Hawkins et al., 1992, 1999; Zucker, Fitzgerald et al., 2000; Mayzer et al., 2001, 2002).
