Caracalla made every free male a citizen of the Roman empire, enfranchising millions of people. Did he do this primarily to increase tax revenue?
All in all, we know very, very little about the so-called Constitutio Antoniniana. What we know is far out of proportion with its supposed importance. This is what the ancient texts have to say about the CA:
In orbe romano qui sunt ex constitutione imperatoris Antonini cives Romani effecti sunt.
All those in the Roman world were made Roman citizens according to a decree of the emperor Antoninus (i.e. Caracalla)
but the new [taxes] which he promulgated and the ten per cent tax that he instituted in place of the five per cent tax applying to the emancipation of slaves, to bequests, and to all legacies; for he abolished the right of succession and exemption from taxes which had been granted in such cases to those who were closely related to the deceased. This was the reason why he made all the people in his empire Roman citizens; nominally he was honouring them, but his real purpose was to increase his revenues by this means, inasmuch as aliens did not have to pay most of these taxes.
That's it. The text of the CA doesn't exist, unless we think that the Digest, which refers to Ulpian as its authority, is quoting a part of the CA. That's enough evidence to suppose that Caracalla extended citizenship to the free peoples of the empire--there's no question that citizenship was assumed for all residents of the empire by the time of Justinian, which is why the Digest quotes Ulpian here--but not enough for much else. We don't know how Caracalla would have done it. A single edict seems likely, but nothing about what Ulpian says actually indicates that, despite how I've translated "ex constitutione." Even the date of the CA is conjectural. We get 212 because Dio mentions the extension of the franchise when he's talking about stuff that happened in 212, but you can see from the passage that there's no indication as to when this actually happened.
You'll notice also that the sources are either very brief or simply don't actually care about the extension of the franchise. Ulpian's passage is too brief to be terribly illuminating, and Dio's not concerned with the CA but rather with Caracalla's financial excess, which he has a lot to complain about. This is where the idea that Caracalla issued the CA in order to generate a greater tax base comes from. But there isn't really much reason to believe Dio on this one. Dio hates Caracalla, and you'll notice that he explicitly denies what must have been the prevailing (or perhaps even official, if the CA was in fact something that was widely publicized, though there's little reason to think that) view, that Caracalla was honoring the peoples of the empire. Instead, says Dio, Caracalla actually had a hidden nefarious motive, which is typical when he talks about Caracalla. Doesn't necessarily means that he's wrong, but it does mean that we ought to suspect him of overemphasizing something that may not have been as important as he suggests.
There are two basic issues with the CA, which is generally accepted as having happened even if there's not necessarily agreement on what it looked like or how it was issued. First, what did it actually do? Second, why was it issued?
The quotation of Ulpian in the Digest is simultaneously specific and vague, and doesn't help us much. Ulpian seems simply to say that anyone living in the empire was made a Roman citizen. He uses a present tense verb, est. Does that mean that anyone living in the empire at any time is a citizen? Like, if I move into the empire permanently from Scythia or Arabia, am I now a citizen? Or does he mean those living in the empire at the time of the edict's proclamation? Dio's summary of the edict has similar issues. A related problem is what citizenship actually means. Ulpian says that these people became cives Romani, Roman citizens. Should be cut and dried, but it isn't. In the past few decades we've found increasingly that Roman citizenship in the provinces was not equivalent across time and space. Inscriptions from Africa and other provinces, for example, refer to viritane grants of citizenship in which the people in question have privileges granted to them specifically that we would normally think were assumed by all citizens, like freedom from tribute. Clearly, then, not all cives Romani were equivalent all the time in all places during the imperial period. And what does this mean for groups like the Latins, which had become a status title rather than a provincial one? Unfortunately we really don't know. Like criminal law, citizenship law basically isn't included in the Digest, since it was completely irrelevant by the time of Justinian, and we have to work almost entirely from inscriptions. And was the grant of citizenship, for example, basically just a title with no real effects? We're not sure. Then there's the further problem that status words, most notably peregrini, appear after the CA when they ostensibly shouldn't. In the later empire peregrini--resident aliens--appear in rather large numbers. Theoretically these people should be citizens. The dominant view today is that the peregrini of the later empire were entire foreign nations like the Goths that were settled in the empire, and that therefore the CA granted citizenship to those inside the empire at the moment of its enactment. But that's pretty speculative, and we don't have enough evidence to be secure on that one.
Dio's assertion that the CA was intended to incorporate a greater tax base seems plausible enough, but there are a lot of holes and unanswered questions here. If many citizens in the Latin west didn't have "full" citizen privileges during the imperial period, then how does the CA ensure that everybody is now paying the inheritance tax that Dio mentions? What happens to the people who have different citizen rights? Moreover, what does this do to the law? Clifford Ando has suggested fairly recently that the CA should be viewed as part of an ongoing process whereby imperial rescripts, centralized (Roman) civil law, and localized provincial legal frameworks were integrated into the structure that became the Digest after several centuries. Seen this way, the CA is part of a centuries-long process of provincial incorporation, a standardization of the rights of citizens and imperial residents. Ando makes a convincing argument, but it's very speculative and relies mostly on theory over direct evidence. And even if Ando's right the CA need not have been enacted with such a purpose explicitly in mind. Most of the parts of the process that he's talking about happened organically without an organized policy, so it's hard to see why the CA would be different.
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