We tested for zygosity differences that might identify limitations to the generalizability of twin data as part of preliminary analyses. Because monozygotic (MZ) twinning occurs at random, MZ twins will represent a near-random sample of genotypes in the general population. In contrast, dizygotic (DZ) twining shows weak associations with factors including maternal age and socioeconomic status (Bulmer, 1970). In preliminary analyses of reproductive and AD onset, cumulative failure curves were estimated using the Kaplan-Meier survivor function (Kaplan & Meier, 1958), with log-rank tests to identify significant differences in equality of survivor functions by gender and cohort.
