Has the proportion of slaves in the Roman Empire varied over its history? Has it always been constant?
A census of citizens and slaves has not survived, so all population guesses are a bit dodgy. However, slaves are like industrial machinery; in fact that’s what a lot of them were. When the economy is booming people invest; in a slump, they sell, or at least stop buying. Again, slaves come from two sources: born at home, or acquired (warfare or trading) abroad. It follows that slaves would be plentiful and cheap following a major war of acquisition. This suggests that the slave population would follow the economic cycle and the recurrent wars on the frontier expanding the empire. If you can find an economic history of Rome and correlate it with the history of Roman warfare, you should be able to work out, roughly, the ups and downs of the slave population. Remember that in general the number of slaves is likely to have risen as the empire rolled on simply because of the increased need for labour – so irrespective of short-term events, the general trend would be upwards for a long time.
