Two early twin studies provide strong evidence for the heritability of acute responses to stimulant drugs (Nurnberger et al. 1982; Crabbe et al. 1983). Twin studies estimate heritability by comparing the concordance rate between monozygotic twins, who share a familial environment and all genes, to dizygotic twins, who share the same environment but only half of their genes. Typically, biometric modeling is used to explain variability due to genetic or environmental effects. Heritability estimates can range from 0 (no variation contributed by genetic sources) to 1 (all variation contributed by genetic sources). Nurnberger et al. (1982) administered d-amphetamine intravenously (0.3 mg/kg) to 13 pairs of monozygotic twins and 3 pairs of dizygotic twins and measured physiological and subjective effects of the drug. Responses to the drug in monozygotic twins were highly concordant for a subjective measure of excitation, as well as growth hormone and prolactin release, suggesting a large genetic component underlying these traits. Consistent with this, Crabbe et al. (1983) administered 10 mg d-amphetamine sulfate to six pairs of monozygotic twins and found less variation in physiological and subjective responses within pairs than between pairs, which suggests these traits are heritable.
