additional contributions by weinberg to statistical genetics included the first estimate of the rate of twinning – realizing that identical twins would have to be same-sex, while dizygotic twins could be either same or opposite sex, weinberg derived the formula for estimating the frequency of monozygotic and dizygotic twins from the ratio of same sex and opposite twins to the total of maternities. weinberg also estimated that the heritability of twinning itself was close to zero.