How many citizens were in each century of the Roman electoral system?

Question

The Centuriate assembly (Comicia centuriata) was arguably the most important assembly of the Roman republic. It was in charge of electing consuls, praetors and censors, of voting laws, declaring war and peace, judging capital cases, etc.

The Centuriate assembly included all Roman citizens, but not all votes carried the same weight. Citizens were organized by centuries according to their role (if any) in the army, and centuries had different sizes but each the same voting power. There were 193 centuries (at least during most of the republic), 18 of equites, the richest citizens able to buy a horse for war, 170 of infantry divided in 5 classes according to the weapons they soldiers could buy themselves, 4 of diverse people, and one of proletarii, too poor to be in the army.

All this information is in Livy (perhaps not as detailed there), on wikipedia, and almost in every modern book on the Roman republic I looked at. Obviously, the centuries of equites were composed of fewer citizens that the centuries of first class infantry, which gave each of the member of an equites century a greater individual voting power, etc...

But a question that I have not seen answered is :

How many citizens was there in each century of the different types?

Of course there is no definite answer because it must have change during the long life of the roman republic. And it may be difficult to know precisely. But has anyone made some serious estimates on the question, say for certain dates like 400BC, 300Bc, 200BC? The question seems important in order to evaluate how biased toward the richest citizens was the centuriate assembly, and the Roman republic as a whole (which of course had other biased institutions, like the senate).

Answer 1

This graph seems to show somewhat of you're looking for:

http://mappinghistory.uoregon.edu/english/EU/EU02-02.html

The graph shows that:

• between 500 BC and 350 BC, there were between 100,000 and 200,000 citizens
• between 350 BC and 225 BC, there were between 200,000 and 300,000 citizens
• There was a 100,000 citizen dip from 225 BC to 175 BC during the Hannibalic wars
• From 175 BC to 100 BC, the citizen population rose steadily from 300,000 to 400,000
• From 100 BC to 75BC, the citizen population spiked from 400,000 to 1 million

I believe these are the official census numbers, which should be taken with a large grain of salt since counts were done in different ways at different times.

I've been finding this kind of information really hard to come by : /

Answer 2

Scholars argue over the very basics of the functioning of the comitia centuriata . Here is the nearest to a hard number I could come up with. The total number of equites in the late Republican/Augustan period is generally thought to be around 10-20,000 (much scholarly squabbling, but most guesses converge around here). The census of 86BC counted 463,000 Roman citizens and the 70BC census (after the Social war expansion) counted 910,000. Since the equites had 18 out of 193 centuries, 1-2% of the electorate had 10% of the vote. But there are lots of other points to consider - only citizens actually in Rome or who came to Rome could vote, we don't know the relative sizes of the tribes, the numbers of juniores compared to seniores, how strong exactly the moral influence of the centuria prerogativa was, etc. In fact, Fergus Millar has challenged the general belief that the poorer Romans were so without electoral and political influence (p.16-17).