Various pivotal stages can be identified in an individual's smoking history, including smoking initiation, conversion from experimenting to established use, development of tolerance, and cessation. Each transition is likely influenced by environmental and genetic factors, some of which are common to all steps, and others that are specific. Modest to high heritability has been reported for the majority of smoking behavior phenotypes (Madden et al. 2004; Horimoto et al. 2012; Loukola et al. 2014), with a study of Finnish adult twins reporting heritability estimates of 0.59 in males and 0.36 in females for age at initiation of smoking (Broms et al. 2006).
