Monozygotic twins have double concordance rate for hysterectomy compared to dizygotic twins. The classic twin study compares monozygotic and dizygotic twins. They used skin grafting to differentiate between monozygotic and dizygotic twins in cattle. Monozygotic (identical) but not dizygotic (fraternal) twins tend to have similar sleep habits. Reversade also researches the genetics of dizygotic and monozygotic twinning in humans. The classical twin design compares the similarity of monozygotic (identical) and dizygotic (fraternal) twins. The vast majority of twins are either dizygotic (fraternal) or monozygotic (identical). It is a common misconception that two placentas means twins are dizygotic. Studies show that there is a genetic proclivity for dizygotic twinning. IVF techniques are more likely to create dizygotic twins. Studies involving humans often compare opposite-sex to same-sex dizygotic twins. Women who have three or more children are also likely to have dizygotic twins. The Yoruba present the highest dizygotic twinning rate in the world (4.4% of all maternities). The two eggs, or "ova", form two zygotes, hence the terms "dizygotic" and "biovular". Women undergoing certain fertility treatments may have a greater chance of dizygotic multiple births.