It is said that Caligula’s exasperated wish was that the people of Rome had only a single neck. That they had a single – and very strongly felt – collective identity is, by contrast, our historical problem. Urban populations at all periods suffer from being treated corporately – as the demos, the many, the mob, the multitude, the masses: under such concepts a sneer lies close below the surface, and the dehumanizing effect of the collective designation has never lost its political point. The difficulty is particularly acute in the case of ancient Rome. The population in question was very large (though for reasons that we shall see, quantification poses serious problems, not just of evidence). Secondly, the Roman elite had every reason to develop the vocabulary of disdain, and has processed almost all the information we possess. Thirdly, there were indeed ways in which the plebs Romana was in reality a corporate entity, and really did cohere as a collectivity, so even when the dismissive perceptions of ancient aristocrats have been allowed for, our analysis still has to penetrate an institutional facade before it can depict and explain the differentiations within the Roman populace.

Our subject-matter in this chapter is the resident population of the city of Rome; but there are two other collectivities that need to be distinguished. The first, the plebs urbana, was a subset of the urban population; it comprised the Roman citizens resident in the city who were not members of the senatorial or equestrian census-categories: it excluded slaves and foreigners (peregrini). The second, the populus Romanus, was the sum of all Roman citizens of whatever status everywhere.

The populus Romanus and the plebs urbana were in early Roman history very nearly co-extensive, but as Rome was involved in increasingly farflung theatres of activity and new citizens outside Rome were included within the body politic, they became widely separated. The populus Romanus had had from an early date an important practical and theoretical standing in the Roman state, producing what could be regarded as a spectacular example of a mixed constitution.