consequences of dependence is generally lacking. Very few adults initiate drug use voluntarily if they have remained drug naïve into adulthood, and tobacco and alcohol industries specifically target youth to recruit new users for this reason. However, abuse and addiction to prescription opiates is also becoming a problem among older adults. A strong genetic component in the etiologies of addictions has been identified, and several addictions are among the most heritable psychiatric disorders (3). Numerous family, twin and adoption studies have provided consistent evidence for the role of genetic factors, by estimating heritability as the fraction of inter-individual differences that can be attributed to genetic differences between individuals. Estimates of the heritability of smoking persistence/dependence vary at 0.4 to 0.8 (reviewed in (4, 5), table 1), while estimates for alcoholism typically range at 0.5 to 0.7 (reviewed in (3, 6), table 1). Similarly, heritability estimates for initiation and/or dependence of illicit drug use have been reported at 0.3 to 0.6, although fewer studies have focused on this (reviewed in (3, 7), table 1). It should be noted that heritability estimates are population- and time-specific, and other factors that can influence the risk of addiction indirectly (such as impulsivity) are also
