Twin pairs (n=3181 pairs) were classified into three groups: pairs concordant for early cannabis use (i.e., both used cannabis before age 18; 4%), pairs discordant for early cannabis use (i.e., one used cannabis before age 18 and the other did not; 9%), and pairs concordant for not using cannabis before age 18 (87%). Pairs discordant for early cannabis use were the focus of cotwin-control analyses using conditional logistic regression. In these analyses, twin pairs are matched for familial background and vary only in that one twin (considered to be the “case”) is an early-user whereas the other twin (i.e., the “control”) is not. The test is whether the “case” is at increased risk of having a particular outcome compared to the “control”. Because the discordant cotwins of early users included both those who had never used cannabis (35%) and those who used cannabis later, two series of conditional logistic regression analyses were conducted: one examining both later-using and never-using cotwins (n=293), and one restricted to the subset of discordant pairs in which the cotwin initiated cannabis use later (n=190). These
