Unequal rate of monozygotic and like-sex dizygotic twin birth: evidence from the Minnesota Twin Family Study.

It is generally believed that in Caucasian populations the rate of monozygotic (MZ) twinning is approximately equal to the rate of like-sex dizygotic (DZ) twinning so that representative samples of like-sex twins should contain approximately equal numbers of MZ and DZ twins. Recent evidence suggests, however, that the rate of DZ twinning in Caucasian populations has declined over the past 50 years so that there are now many more MZ than like-sex DZ twin births (Jeanneret and MacMahon, 1962; James, 1972; Mosteller et al., 1981; Doherty and Lancaster, 1986; Lykken et al., 1990). We report additional evidence of a higher rate of MZ than like-sex DZ twinning from Minnesota for the birth years 1971-1984. The convergence of evidence thus suggests that the observation of a greater number of MZ than DZ twins in a volunteer twin sample can no longer necessarily be taken as a sign of ascertainment bias.