Finally, we examine a more refined series of models by estimating the heritability of regular smoking for each year of birth in our sample. Because of small sample sizes for each birth year in our study, we calculate time-specific estimates for a moving sample with a window of ± four years. For example, an estimate for 1930 includes individuals born between 1926 and 1934. We do this for each year between 1922 and 1968. These models, summarized in Table 4, include controls for the respondent’s age and gender and measures of the equal environments assumption.5 In order to gauge the statistical significance of the estimates, we provide the bootstrapped confidence interval for the yearly estimates. Each year-zygosity model was run 200 times using sample sizes equivalent to the empirical size with replacement. The confidence intervals are, therefore, empirical confidence intervals for the 10th and the 190th ranked value from each distribution. These bootstrapped models were performed using the “coxph” package in the R 2.9.0 statistical program (R Development Core Team 2009). If the confidence interval crosses over zero, then we
