Much of what we know about illegitimacy concerns the wealthier classes. As far as we can tell, it was not that much of an issue during the Republic. Recorded examples of illegitimate children are few, and it’s likely that most such children were aborted, exposed, or simply raised as though they were the legitimate children of the mother’s husband. By the time of the Empire illegitimacy was a bit more of a concern. Augustus and other emperors created laws criminalizing adultery and regulating inheritance by illegitimate children. How strongly these were enforced though, depended on the emperor.
Originally, illegitimate children weren’t entitled to any inheritance unless they were named in a will. Eventually, they were allowed to inherit from their mothers even without a will, where before property had gone first to legitimate children, then to her male relatives (brothers, fathers, uncles, cousins). At this point the mother of an illegitimate child was also allowed to inherit from her child if they died intestate.*
Among the lower classes, one’s status as freeborn, freed, or unfree was far more important and relevant than legitimacy. In the case of a child born outside of a valid Roman marriage, the child followed the status of the mother. The child of a slave was born a slave. The child of a free woman** was freeborn. This would cause some concern with the rising numbers of free women living as the contubernales of highly influential slave men. The child’s illegitimacy was only really important here in that it meant they inherited their mother’s free status rather than being unfree like their father.
*Without making a will.
**Or a woman freed and subsequently enslaved again during her pregnancy. Legally, at least. How that worked out in practice was most likely another matter entirely.
Edwards, Catharine. The Politics of Immorality in Ancient Rome. Cambridge: Cambridge University Press: 1993.
Rawson, Beryl. The Family in Ancient Rome: New Perspectives. Ithaca, NY: Cornell University Press, 1986.
McGinn, Thomas A.J. Prostitution, Sexuality, and the Law in Ancient Rome. New York: Oxford University Press, 1998.