Tests for zygosity differences in AD and age at first childbirth largely support generalizability of twin data. Zygosity is unrelated to AD in Cohort I [women: χ2 (df) = 2.63 (1), p =.10; men: χ2 (df) = 3.52 (1), p =.06] and for women in Cohort II [women: χ2 (df) = 3.24 (1), p =.07]. There is a slight difference by zygosity for men in Cohort II [χ2 (df) = 4.04 (1), p =.04]. For both sexes, differences by zygosity in the cumulative probability of first childbirth are nonsignificant in Cohort I [women: χ2 (df) =.07 (1), p =.79; men: χ2 (df) =.03 (1), p =.86] and Cohort II [women: χ2 (df) =.03 (1), p =.86; men: χ2 (df) = 0 (1), p = 1.00]. Cox Analyses of Reproductive Timing and AD
