not endorsing four or more withdrawal symptoms within 24 hours of stopping or cutting down on cigarettes for the initially chosen episode were asked: “Has there been a time when you had 4 or more of the [nicotine withdrawal] problems occur during the first 24 hours after you cut down or stopped smoking cigarettes?”, if ‘yes’, the subject was cycled through the questions on nicotine withdrawal symptoms again. Individuals were classified according to whether they met criteria for DSM-IV nicotine withdrawal (NW) using the substance dependence definition, i.e. endorsement of four or more symptoms within 24 hours of quitting/reducing use of cigarettes, or endorsement of smoking cigarettes, or any other tobacco product, nicotine substitute or medication to relieve or avoid withdrawal symptoms (APA, 1994). For the affected sib-pair linkage analyses, all NW affected sib-pairs reported smoking 100 or more cigarettes lifetime. Thus for all phenotypic analyses reported, individuals who reported smoking fewer than 100 times lifetime were set to missing (0.01% of smokers). The use of 100 or more cigarettes lifetime is widely used to define a lifetime history of adult cigarette smoking (CDC, 2006). In the broader Australian sample, 51.9% of siblings with a history of smoking met criteria
