As with other ancient self-governing citizen bodies, the populus Romanus (the Roman people – an idea that was defined by citizenship) restricted political participation to adult citizen males (actual office holding was further restricted to adult citizen males with military experience, Plb. 6.19.1-3). And we should note at the outset that citizenship was stratified both by legal status and also by wealth; the Roman Republic openly and actively counted the votes of the wealthy more heavily than those of the poor, for instance. So let us avoid the misimpression that Rome was an egalitarian society; it was not.

The most common way to become a Roman citizen was by birth, though the Roman law on this question is more complex and centers on the Roman legal concept of conubium – the right to marry and produce legally recognized heirs under Roman law. Conubium wasn’t a right held by an individual, but a status between two individuals (though Roman citizens could always marry other Roman citizens). In the event that a marriage was lawfully contracted, the children followed the legal status of their father; if no lawfully contracted marriage existed, the child followed the status of their mother (with some quirks; Ulpian, Reg. 5.2; Gaius, Inst. 1.56-7 – on the quirks and applicability in the Republic and conubium in general, see S.T. Roselaar, “The Concept of Conubium in the Roman Republic” in New Frontiers: Law and Society in the Roman World, ed. P.J. du Plessis (2013)).

Consequently the children of a Roman citizen male in a legal marriage would be Roman citizens and the children of a Roman citizen female out of wedlock would (in most cases; again, there are some quirks) be Roman citizens. Since the most common way for the parentage of a child to be certain is for the child to be born in a legal marriage and the vast majority of legal marriages are going to involve a citizen male husband, the practical result of that system is something very close to, but not quite exactly the same as, a “one parent” rule (in contrast to Athens’ two-parent rule). Notably, the bastard children of Roman women inherited their mother’s citizenship (though in some cases, it would be necessarily, legally, to conceal the status of the father for this to happen, see Roselaar, op. cit., and also B. Rawson, “Spruii and the Roman View of Illegitimacy” in Antichthon 23 (1989)), where in Athens, such a child would have been born a nothos and thus a metic – resident non-citizen foreigner.

The Romans might extend the right of conubium with Roman citizens to friendly non-citizen populations; Roselaar (op. cit.) argues this wasn’t a blanket right, but rather made on a community-by-community basis, but on a fairly large scale – e.g. extended to all of the Campanians in 188 B.C. Importantly, Roman colonial settlements in Italy seem to pretty much have always had this right, making it possible for those families to marry back into the citizen body, even in cases where setting up their own community had caused them to lose all or part of their Roman citizenship (in exchange for citizenship in the new community).

The other long-standing way to become a Roman citizen was to be enslaved by one and then freed. An enslaved person held by a Roman citizen who was then freed (or manumitted) became a libertus (or liberta), by custom immediately the client of their former owner (this would be made into law during the empire) and by law a Roman citizen, although their status as a freed person barred them from public office. Since they were Roman citizens (albeit with some legal disability), their children – assuming a validly contracted marriage – would be full free-born Roman citizens, with no legal disability. And, since freedmen and freedwomen were citizens, they also could contract valid marriages with other Roman citizens, including freeborn ones […]. While most enslaved people in the Roman world had little to no hope of ever being manumitted (enslaved workers, for instance, on large estates far from their owners), Roman economic and social customs functionally required a significant number of freed persons and so a meaningful number of new Roman citizens were always being minted in the background this way. Rome’s apparent liberality with admission into citizenship seems to have been a real curiosity to the Greek world.

These processes thus churned in the background, minting new Romans on the edges of the populus Romanus who subsequently became full members of the Roman community and thus shared fully in the Roman legal identity.

Bret Devereaux, “Collections: The Queen’s Latin or Who Were the Romans, Part II: Citizens and Allies”, A Collection of Unmitigated Pedantry, 2021-06-25.