a ‘person year’ data set using SAS (37). Data were constructed such that each line of data represented a single year of life for each case. CSA was coded as absent in each year prior to age of onset and present for each subsequent year. Zygosity was coded with dizygotic twin pairs as the reference group. Analyses were conducted in Stata (38) using the Huber-White correction to adjust for the non-independence of observations in twins. Model 1 included only CSA. Co-twin early use and the interaction between co-twin early use and zygosity were added in Model 2.
