Family, twin, and adoption studies have consistently reported that genetic factors contribute to dependence behaviour and relapse after treatment through interactions with environmental factors [51-54]. Studies of probands with substance related disorders and their family members have reported significant associations between various drug groups and their genetic and environmental determinants of dependence [55]. These studies have provided the evidence to determine the extent of genetic contribution to dependence through estimation of heritability factor (h2) [54, 55]. The heritability is characterized by three types of factors: genetic factors (A), shared family environmental factors (C), and random or unique environmental factors (E) [56, 57].
