Finally, if there was a significant association between a tobacco use and a SRB variable, we further explored the association by controlling for confounds shared by the twins using a discordant twin design. Such within-pair analysis controls for familial factors constant within twin pairs, such as environmental factors shared by the co-twins (e.g. family environment) and shared genetic liability for both tobacco use and SRB (Kujala et al., 2002). Thus, if an analysis among twin pairs discordant for tobacco and SRB provides similar results as provided by the individual-based analysis, the association between tobacco and SRB is not confounded by underlying familial (shared genetic or environmental factors) liability. All analyses were conducted using the statistical software Stata (version 13) (StataCorp. 2013).
