Models including variables coding for genetic effects, suggest heritable covariation contributes little to observed associations between reproductive timing and AD. For women in Cohort I, the interaction between zygosity and co-twin AD is nonsignificant (HR = 1.35, 95% CI:.51 – 3.62), as is the interaction of zygosity, cotwin AD, and cotwin sex (HR = 1.45, 95% CI:.62 – 3.39). In Cohort II, where significant associations between reproductive timing and AD are observed for both sexes (for men, in adjusted models only), the interaction between zygosity and co-twin AD is nonsignificant for both women (HR =.84, 95% CI:.41 – 1.70) and men (HR = 1.09, 95% CI:.29 – 4.08). Nonsignificant interactions of zygosity, cotwin AD and cotwin sex are also observed for women (HR =.97, 95% CI:.49 –1.93) and men (HR = 1.26, 95% CI:.52 –3.04) in Cohort II.
