Extensive data from classical genetic studies indicate that vulnerabilities to substance dependence are complex traits with strong genetic influences. These genetic influences are largely shared by abusers of different legal and illegal addictive substances [1-4]. Nicotine dependence can be defined by Fagerstrom Test for Nicotine Dependence [5] or by DSM (diagnostic and statistical manual) criteria. Defined in either fashion, twin studies of nicotine dependence support strong heritability in the range of 40 – 70% [6-9].
