The spread of Roman citizenship, 14-212 CE: quantification in the face of high uncertainty

This paper borrows an analytical method from the sciences to solve an important problem in Roman history, which has long seemed intractable: estimating the proportion of provincials who had Roman citizenship before Caracalla’s general grant of 212/213 ce. The scale of enfranchisement in the early empire has important ramifications for our understanding of the significance of Roman citizenship in that period and the impact of Caracalla’s grant. Yet it has so far eluded quantification entirely. Previous efforts have focused on counting names on inscriptions and other documents and failed to produce any robust conclusions. The problem demands a new approach. This paper starts from the fact that we know that there was a limited number of mechanisms by which new citizens were created and shows that there are limits to the number of citizens those mechanisms can have created over two centuries. There is of course considerable uncertainty about many of the relevant variables, but this can be managed thanks to well-established probabilistic techniques for the estimation and propagation of uncertainty. Given what we already know about the mechanisms of enfranchisement and the demography of the provinces, it is extremely unlikely that more than one third of the free population of the provinces were Roman citizens on the eve of Caracalla’s grant.

These are the preliminary results of an ongoing research project. Future publications will refine the estimate, provide more detailed discussion of the underlying assumptions and further explore the historical implications of the result. Here the focus is on methodology, demonstrating that the uncertainty about the individual mechanisms of enfranchisement does not preclude a useful quantitative estimate of the overall level of enfranchisement. The analytical method described here could be a useful tool in attempts to solve many other historical problems that seem similarly beset by insuperable uncertainty.