From the middle of the third to the end of the second centuries B.C., the adult male population was estimated to have risen from about 200,000 to 400,000 individuals. Those numbers, however, don't jibe with censuses organized by the first emperor Augustus in the first centuries B.C. and A.D., which showed a population that had increased to about 4 million to 5 million males.
While the granting of citizenship to allies on the Italian peninsula accounts for some of the increase, there is still an estimated unexplained doubling or tripling in the Roman population before the first Augustan census in 28 B.C. Just what accounts for that increase is a matter of intense debate.
One camp explains the discrepancy by suggesting that the Empire began counting women and children in the census. While this would account for the relative increase, it would actually imply an overall decline in the population of Rome and there are no suggestions that the entire populace was counted in historical records.
On the other side of the debate are those who suggest that the population simply boomed. This would mean that the Roman Empire — and other premodern societies — achieved much higher economic output than previously supposed. It would mean that Roman history as it is now understood would have to be rewritten.
The resolution of this debate was found in buried treasure. Two university professors, Peter Turchin of the University of Connecticut and Walter Scheidel of Stanford University, turned their attention to coin hoards — treasure that families buried during times of conflict and then were unable to later recover. Turchin and Scheidel reasoned that by compiling a record of such hoards, and the times in which they were buried, they could learn a great deal about population trends.
Their findings? The population of Rome declined after 100 B.C. and the estimates of a population of 4 to 5 million people was, in fact, the entire population of the Roman Empire and not simply a count of the male population. As LiveScience.com quotes Scheidel: "This may seem like an arcane dispute, but it isn't really because the difference is so large — 200 percent.... This model is much more consistent with the low count. I'm not sure that by itself it has absolutely proven it, but it certainly provides additional evidence for the low-count hypothesis." What is interesting is that it also means that if Turchin and Scheidel are correct in their analysis, even as Rome grew in wealth and power, the population of the empire fell. The demographic winter that is afflicting our powerful, wealthy but also increasingly spiritually empty society may echo the experience of an early culture that moved from being a republic to a "multi-cultural" empire.