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 mon ozygotic (MZ) twinning is approximately equal to the rate of like-sex dizygotic (DZ) twinning so that representative samples of like-sex twi ns should contain approximately equal numbers of MZ and DZ twins. Rece nt evidence suggests, however, that the rate of DZ twinning in Caucasi an populations has declined over the past 50 years so that there are n ow 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 19 71-1984. The convergence of evidence thus suggests that the observatio n of a greater number of MZ than DZ twins in a volunteer twin sample c an no longer necessarily be taken as a sign of ascertainment bias.