[Proof of illegitimacy and identifying the true father after 70 years].

A man born during the First World War doubted his legitimacy and believed his true father to be his mother's (since deceased) cohabitant. His and his sister's findings in less than or equal to 24 blood group systems produced a probability of sibship of W = 33%. The probability of half-sibship between the proband and two of the cohabitant's legitimate children was W = 82%, between him and two other children W = 70%, and between him and yet another two children W = 73%. These values raised doubts about the legitimacy of the cohabitant's six children. By means of serostatistics, using the blood group findings of 8 individuals it was established that it is "highly probable" that two of the six children are not the legitimate children of the cohabitant, but that their father is an unknown man. It was "practically proven", W = 99.97%, that the four legitimate children of the cohabitant are half-siblings of the proband, i.e. they all have the same father. The probability that the initial situation is correct--the proband and the cohabitant's six children are all legitimate-is "practically refuted": W = 0.03%. The probability that the position established on the basis of the blood group findings--the proband is the child of the cohabitant; two of the cohabitant's children are in fact illegitimate--is "practically proven": W = 99.97%.