Given the nature of our evidence, we can employ only slightly more sophisticated methods of calculating the population of Rome than Elagabalus, who proclaimed the greatness of the city on the basis of its cobwebs. However, while we must be content with orders of magnitude rather than exact figures, a consideration of population is central to our understanding of the city of Rome. The number and social composition of the inhabitants impacts upon many areas of urban life; for example, housing, leisure activities, sanitation, life expectancy, family structure, water and food supply, production and commerce, and politics. The demographic dynamic of a city’s population also has a significant effect on that city’s economic infrastructure. There is, for example, a critical relationship between voluntary migration and an urban economy, and this paper explores that particular relationship in the city of Rome. For much of its history, but particularly in the late republic and the principate, Rome was a city of free migrants, with a population profile that was in a constant state of flux. As a ‘dynamic force’, migration does not merely respond to demographic, economic or social developments, but creates them. The study thus begins with a consideration of the population of Rome and the phenomenon of migration. The relationship between free migration, urbanisation and economic development is then explored. It is argued that the particular social and institutional framework of Rome limited the economic opportunities for the freeborn inhabitants of the city, particularly new migrants, both temporary and permanent. Drawing upon a combination of ancient evidence, migration theory, economic models and studies of comparative cities, this paper contends that many of the inhabitants of Rome remained trapped in absolute, structural poverty.

No literary testimony or inscription records a figure for the entire population of ancient Rome. Although Rome took regular censuses of its citizens, these refer primarily to adult male citizens, and the birth and death records kept in the city do not survive. In any case, even if our ancient sources had diligently recorded statistical population data, it is doubtful that there was ever any accurate means of recording a large and fluctuating urban population. In fact, for the metropolises of Roman Egypt, where we know more about the workings of census taking, the data are extremely susceptible to inaccuracies as a result of hidden or mobile individuals of both sexes and various ages, particularly young men. The problem of recording such a mobile population can also be demonstrated by recent experience in the developing world, where many of the population data are flawed and inaccurate, particularly in sub-Saharan Africa. This can, however, be used to draw some conclusions about the general order of magnitude of the population. Similarly, despite our lack of accurate data, most modern scholars now accept that at its greatest extent, in the late republic and the principate, the population of Rome was somewhere in the region of 1 million inhabitants.