From studies that compare the concordance of smoking among identical twin pairs to that of fraternal twin pairs, researchers estimate that roughly 50 to 60 percent of the variance of regular smoking is due to genetic factors (Carmelli et al. 1992; Sullivan and Kendler 1999; Hall, Madden, and Lynskey 2002; Li et al. 2003). However, by comparing reported tobacco use among same-sex twin pairs across three birth cohorts (1910–1924–1925–1939, and 1940–1958), Kendler et al. (2000) demonstrate that heritability estimates are subject to change over time—a result that makes a single figure misleading. Among the first cohort of women they studied, none of the variance in tobacco use came from genetic factors, but by the third cohort, the heritability for regular tobacco use reached nearly 60 percent.
