Emmet Scott has published an article at the New English Review called "The Role of Infanticide and Abortion in Pagan Rome's Decline." Some excerpts:
The crisis of the third century naturally became the subject of intense debate amongst historians. Nowadays it is often regarded as having an economic origin, and scholars talk of inflationary pressures and such like. This may be partly true; but what seems undeniable is that the real problem lay deeper. There is now little dissention on the belief that by the year 100 the population of the Empire had ceased to grow and had begun to contract. ...The author goes on to note that rather than contributing to the decline of the Roman Empire, both the cosmopolitan Byzantine Empire and the more rural Western Empire (or what little of it remained) were saved by Christianity ... for awhile. He concludes:
From the same period archaeologists have noted not only the cessation of major new building but also the demolition and recycling of existing monuments. (See eg Peter Wells, Barbarians to Angels (New York, 2008), pp. 109-10) There appears also in the urban settlements of temperate Europe a layer of dark humic soil, sometimes more than a meter thick, containing cultural debris – pottery, bones of butchered animals, glass fragments, etc – mixed into it, covering occupational remains of earlier centuries. “The dark earth,” says one historian, “has been found to contain remains of timber-framed, wattle-and-daub huts, along with sherds of pottery and metal ornaments datable to the late Roman period. These observations demonstrate that people who were living on the site were building their houses in the traditional British [and north European] style rather than in the stone and cement fashion of elite and public Roman architecture.” (Ibid., pp. 111-12) “What are we to make of these two major changes reflected in the archaeology?” the same writer asks. He concludes that, “After a rapid growth in the latter part of the first century... [there was] a stoppage in major public architecture and a reverse of that process, the dismantling of major stone monuments, at the same time that much of the formerly urban area seems to have reverted to a non-urban character.” (Ibid., p. 112)
What could have caused such a dramatic and sustained demographic collapse? ... what has emerged over the past fifty years is a picture of a Roman Empire unfamiliar to most students of classical civilization. It is picture of a world immersed in decadence, squalor and brutality.
Life in a Roman city, it seems, was anything but comfortable. The image of the good life of centrally-heated villas with mosaic floors and marble pillars – the image generally presented to the public in guidebooks and documentaries – was of course far from typical. Much new research has been done on the living conditions of ordinary Romans in the last fifty years, and what has emerged is the picture of a life of almost unimaginable squalor. The cities, by modern standards, were packed: people lived in appallingly confined spaces. In Rome, the great majority of the poor inhabited multi-story apartment blocks named insulae (“islands”), which were little more that multi-story slums. They were also death-traps. Several Roman writers noted that the most frequently heard sound in the city was the roar of collapsing insulae. They were constructed of the cheapest materials, and their occupiers rarely had any warning of their impending disintegration. The streets around these insulae contained a central channel into which the inhabitants threw their sewage. The whole city stank, summer and winter, and so great was the stench that even the rich, in their exclusive areas, could not avoid contact with it. Hence the annual retreat in the springtime to their summer residences in the countryside.
As might be imagined, deadly epidemics were commonplace, and the failure of the ancients to understand the pathology and spread of infections led to a plethora of pandemics which wiped out millions.
Crime too was of epidemic proportions; and a society which exacted the death penalty for minor offences offered no real deterrent against more serious crimes such as murder.
The sheer savagery of Roman attitudes is of course already well known, and we need not labor the obvious fact that people who could watch other human beings being torn to shreds by wild beasts for “entertainment” were of a very low spiritual state. The institution of slavery, by its very existence, had a corrupting effect on attitudes, and slaves, as the property of their owners, could be exploited in whichever way their owners wished. All of them, both male and female, were the sexual playthings of their masters, and must submit to the sexual demands of their owners at any time or place. The sex “industry” was a major employer, as excavations at Pompeii, Herculaneum, and numerous other ancient cities have revealed only too graphically.
As might be imagined, a society which harbored such attitudes did not shrink from taking drastic measures to deal with the unwanted issue of casual liaisons, and the practice of infanticide was widespread and commonplace in the classical world. (See eg. William V. Harris, “Child Exposure in the Roman Empire,” The Journal of Roman Studies, Vol. 84 (1994)) Official Roman documents and texts of every kind from as early as the first century, stress again and again the pernicious consequences of Rome’s low and apparently declining birth-rate. Attempts by the Emperor Augustus to reverse the situation were apparently unsuccessful, for a hundred years later Tacitus remarked that in spite of everything “childlessness prevailed,” (Tacitus, Annals of Imperial Rome, iii, 25) whilst towards the beginning of the second century, Pliny the Younger said that he lived “in an age when even one child is thought a burden preventing the rewards of childlessness.” Around the same time Plutarch noted that the poor did not bring up their children for fear that without an appropriate upbringing they would grow up badly, (Plutarch, Moralia, Bk. iv) and by the middle of the second century Hierocles claimed that “most people” seemed to decline to raise their children for a not very lofty reason [but for] love of wealth and the belief that poverty is a terrible evil. (Stobaeus, iv, 24, 14) Efforts were made to discourage the practice, but apparently without success: the birth-rate remained stubbornly low and the overall population of the Empire continued to decline.
A major and exacerbating factor in the latter was the fact that baby girls seem to have been particularly unwanted. ...
... In addition to infanticide the Romans also practiced very effective forms of birth control. Abortion too was commonplace, and caused the deaths of large numbers of women, as well as infertility in a great many others, and it has become increasingly evident that the city of Rome never, at any stage in her history, had a self-sustaining population, and numbers had continuously to be replenished by new arrivals from the countryside. (For a discussion, see Rodney Stark, The Rise of Christianity: A Sociologist Reconsiders History (Harper Collins, 1996), pp. 95-128)
... Taking this into account, several writers over the past few decades have suggested that Rome’s adoption of Christianity in the fourth century may have had, as one of its major goals, the halting of the empire’s population decline. Christians had large families and were noted for their rejection of infanticide. In legalizing Christianity therefore Constantine may have hoped to reverse the population trend. He was also, to some degree, simply recognizing the inevitable. (Ibid., pp. 95-128) By the late third century Christians were already a majority in certain areas of the East, most notably in parts of Syria and Asia Minor, and were apparently the only group (apart from the Jews) registering an increase in many other areas. This was achieved both by conversion and by simple demographics. The Jews too, by that time, formed a significant element in the empire’s population – and for the same reason: They, like their Christian cousins, abhorred the practice of infanticide and abortion. It has been estimated that by the start of the fourth century Jews formed up to one tenth of the Empire’s entire population. Whether or not Constantine legalized Christianity therefore, it would appear that in time the Empire would have become Christian in any case.
... It has became increasingly clear over the past half century that much more of the heritage of Rome survived than had hitherto been imagined, and that, under the influence of Christianity, Roman civilization flourished both in the East and in the West during the sixth and early seventh centuries.
How the heritage of Rome was ultimately destroyed is another story entirely, but it occurred in the first half of the seventh century and was directly related to the rise of Islam.