Inclusion of samples with lower and higher levels of tobacco use may have increased our ability to detect an empirically significant linkage peak associated with nicotine withdrawal. Among sibling pairs concordant for DSM-IV NW, the range of number of cigarettes smoked per day (cpd) during peak lifetime use varies across subgroups stratified by sex and society, with the Finnish women of pairs concordant for NW at the lower end of the distribution (Finnish women: mean cpd = 18.6) and Australian men of concordant sib-pairs on the upper end of the severity continuum (Australian men: mean cpd = 29.2; Finnish men: mean cpd = 24.3; Australian women: mean cpd = 25.6); suggesting that Finnish women, who are smoking at a lower level, but still part of a sib-pair concordant for NW, might be particularly informative in identifying genetic loci that may contain genes contributing independently to NW vulnerability. Across societies, women were significantly more likely to report depressed mood as a symptom of nicotine withdrawal than men, which appears to be a common phenomenon (Pergadia et al., 2006a). One other notable
