To establish the genetic influences on smoking we use a variety of different techniques. All of these methods rely on comparing identical twins and same-sex fraternal twins to examine the genetic influences on regular smoking for the full sample and separately across birth cohorts. The birth cohorts are defined as follows: 1920–1939; 1940–1949; 1950–1959; and 1960–1970.3 Our first demonstration of genetic influences on regular smoking comes from a comparison of concordance for regular smoking among MZ pairs and DZ pairs; evidence of genetic influence is found when the concordance among MZ pairs is significantly higher than the concordance among DZ pairs. These results are presented in Table 1.
