Numerous family, adoption and twin studies have identified the significant role of heritable influences on individual differences in addiction. Results from twin studies suggest that 33–71% of the variation in liability to nicotine dependence7, 8, 9, 10 can be attributed to heritable influences, while 48–66% of the variation in alcohol dependence11, 12, 13, 14 is heritable. Similarly, a recent meta-analysis of eight twin studies reported heritability estimates of 51–59% for cannabis addiction.15 Heritability estimates for cocaine use disorders range from 42 to 79%, with the lower estimates reported for females.16, 17, 18 Two large-scale studies have examined opioid addiction. Kendler et al.16 reported that 23% of the variation in opioid addiction in men was attributable to genetic factors, whereas Tsuang et al.19 reported a considerably higher estimate of 54% in male Vietnam Era twins. There have also been two major twin studies of disordered gambling, with consistent evidence for heritable (49%) variation.20, 21 Across these studies, there has been no consistent evidence for differences in the magnitude or nature of heritable influences on addiction in men and women. However, two important factors have been found to contribute to variation in heritability:
