Relative frequencies and sex distributions of human multiple births.

Hellin (1895) published evidence indicating that the relative frequencies of multiple births conform to a simple mathematical relationship which may be expressed thus: The frequency of triplet maternities is equal to the square of the frequency of twin maternities, and the frequency of quadruplet maternities is equal to its cube. Stated in general terms, if / is the frequency of twin litters, the frequency of litters of size n is equal to fn~l. This expression has since been tested many times. Although discrepancies between predicted and observed values have been noted (Strandskov, 1945), they appear not to have dispelled belief in the validity of the relationship which, it would appear, has now acquired the status of a "law".