Loss of population
"Still another economic cause of the fall of the empire was the decline in population. The historian Seeley says that the empire perished for lack of men.4 This failure in population resulted in part from slavery, crushing taxation, and the practice of celibacy, and in part from the waste of life caused by constant wars, by plagues, and by the mere contact of civilization with barbarism.5"
"No other industrial system depletes population so rapidly as does slavery. It undermines the family, and at the same time wears out men with a rapidity and ruthlessness not exceeded even by the military system in times of war. In these direct, and in many other indirect ways, slavery helped to thin the population of the empire, and to lay it open to the invasions of the barbarians."
After slavery, the intolerable burden of imperial taxation was perhaps the most prominent cause of the depopulation of the empire. Thousands of the oppressed provincials fled across the frontiers and sought an asylum among the barbarians. Life outside the pale of civilization had become preferable to life within.
Another cause of the decline in population was the singular aversion that the better class of the Romans evinced to marriage. We meet during the period of the empire with a crowd of imperial edicts dealing with this subject. Penalties and bounties, deprivations and privileges, entreaties and expostulations are in turn resorted to by the perplexed emperors, in order to discourage celibacy and to foster a pure and healthy family life. But all was in vain. The marriage state continued to be held in great disesteem (par. 313). And Christianity instead of correcting the evil, rather made matters worse; for just now the teachings of the monks were persuading vast multitudes of the superior sanctity of the solitary or the monastic life, and thereby 5 Ibid., p. 58
filling the deserts of Egypt and the monasteries of all lands with men who believed that they could best live the higher life by freeing themselves of all family and social cares and duties.
To these various agencies of depopulation must be added that of domestic and foreign wars. The many bloody struggles between the numerous aspirants for the imperial dignity, and the constant fighting of the legions in the defence of the frontiers, had an exhausting effect upon the empire. The flower of the Roman race was swept away by the accidents of war, and the gaps in the ranks of the legions could be filled only by recruits from among the barbarians.
Furthermore, during the later centuries of the empire, plagues of extraordinary virulence desolated its provinces. These visitations can be compared to nothing in the following centuries save the terrible pestilence of the Black Death, which in the fourteenth century destroyed from a third to a half of the population of Europe. What made these earlier visitations so much more fatal to society was the fact that the springs of recuperation had then been fatally impaired.
What part in this process of depopulation may be assigned to the last of the causes we have enumerated, namely, the contact of civilization with barbarism, it would be difficult to say. It is a fact that there are races to-day, like the American Indians and the South Sea Islanders, that are melting away from mere contact with a civilization which they cannot or will not assimilate. In the same way, Seeley maintains, in Spain, in Gaul, in Britain, and in the Danubian provinces of the empire, the barbarian races wasted away in the presence of the superior Roman culture which they could not at once make their own.
The signs of the growing depopulation of the empire were to be seen on every side in the ruin-strewn sites of once populous and flourishing cities like Carthage, Corinth, Megalopolis, the Boeotian Thebes and Palmyra. Vast territories formerly astir with life and carefully tilled had reverted to a condition of primitive wildness.
The policies of the emperors, such as bounties on marriage, gifts of land in waste districts to men of families, the wholesale settlement of barbarian tribes in the empty provinces, and similar measures, bear pathetic testimony to the alarming condition of the empire and the unremitting efforts of the emperors to arrest the downward movement of society.
Rome, Its Rise and Fall By Philip Van Ness Myers 1901