2,181. Identifying the Father

Hilchos Issurei Biah 15:14

Let’s say that an unmarried woman participated in premarital relations and claims that her child is the son of Person X. If that person is of kosher lineage, the child is also deemed kosher but the woman’s word is not sufficient for the child to be legally declared that man’s son. The Rambam opines that we give weight to the mother’s words and, because of the doubt, the child may not marry the relatives of his purported father. If Person X is a mamzer, we don’t accept the woman’s word to deem the child a definite mamzer, as has been explained. Rather, we consider this child to be a doubtful mamzer.

Hilchos Issurei Biah 15:15

If a man says regarding a child presumed to be his that the boy is a mamzer, his word is accepted. If the son has children, his word is not accepted because the Torah only accepts his word regarding his own son alone as per Deuteronomy 21:17, “He must acknowledge the firstborn, the son of the hated wife.” We see from this that he must make his children known to others.