Population Dynamics Involved in the Decline and Collapse of Societies and Civilizations
Discussion
Declining birth rates as a factor in civilizational decline:
From the Wikipedia:
"Writing in The Histories, the Greek historian Polybius largely blamed the decline of the Hellenistic world on low fertility rates. He asserted that while protracted wars and deadly epidemics were absent, people were generally more interested in "show and money and the pleasures of an idle life" than in marrying and raising children. Those who had children, according to him, had no more than one or two, with the express intention of "leaving them well off or bringing them up in extravagant luxury."[70][71] However, it is difficult to estimate the actual fertility rate of Greece at the time because Polybius did not provide any data for analysis but gave only a narrative that likely came from his impression of the kinds of Greeks with whom he was familiar: the elites, rather than the commoners. Otherwise, the population decline would have been abrupt. Nevertheless, the Greek case parallels the Roman one.
But since more plenteous honor has come to planes that yield a sterile shade, than to any three, we fruit-bearers (if as a nut tree I am counted among them) have begun to luxuriate in spreading foliage. How apples grow not every year, and injured grapes and injured berries are brought home: now she that would seem beautiful harms her womb, and rare in these days is she who would be a parent.
By around 100 BC, the notion of romantic love started becoming popular in Rome. In the final years of the Roman Republic, Roman women were well known for divorcing, having extramarital affairs, and being reluctant to bear children. Viewing that as a threat to the social and political order and believing that the Roman upper-class was becoming increasingly cosmopolitan and individualistic, upon the establishment of the Roman Empire, Caesar Augustus introduced legislation designed to increase the birthrate. Men aged 20 to 60 and women aged 20 to 50 were legally obliged to marry, and widowed or divorced individuals within the relevant age range were required to remarry. Exemptions were granted to those who had already had three children in the case of free-born people and four in the case of freed slaves. For political or bureaucratic office, preference was given to those with at least three legitimate children. Diminished inheritance rights awaited those who failed to reproduce. In a speech to Roman nobles, he expressed his pressing concern over the low birthrates of the Roman elite. He also said that freed slaves had been granted citizenship and Roman allies given seats in government to increase the power and prosperity of Rome, but the "original stock" was not replacing itself and leaving the task to foreigners. Roman poet Ovid shared the same observation.
However, Augustan pro-natal policies proved unsuccessful. All that they did was fuel nostalgia and disdain for the present and went no further than reaffirming the past-oriented, rural, and patriarchal values of Imperial Rome. Like their Greek counterparts, Roman elites had access to contraception, though that knowledge was lost to Europe during the Middle Ages and the Early Modern Period, and so could enjoy sexual intercourse without having to rear additional children. In other words, people of high socio-economic class of the Greco-Roman world were able to control their own fertility. Also, that ability likely trickled down to the lower classes. In any case, the result was predictable. The absence of modern medicine, which would have extended life expectancy, caused their numbers to start shrinking. Moreover, population decline coincided with people being less religious and more questioning of traditions, both of which contributed to falling fertility as more and more people came to the conclusion that it was up to them, rather than the gods, on how many children they had.
Other population imbalances may occur when low fertility rates coincides with high dependency ratios or when there is an unequal distribution of wealth between elites and commoners, both of which characterized the Roman Empire.
Several key features of human societal collapse can be related to population dynamics. For example, the native population of Cusco, Peru at the time of the Spanish conquest was stressed by an imbalanced sex ratio.
There is strong evidence that humans also display population cycles. Societies as diverse as those of England and France during the Roman, medieval, and early modern eras, of Egypt during Greco-Roman and Ottoman rule, and of various dynasties in China all showed similar patterns of political instability and violence becoming considerably more common after times of relative peace, prosperity, and sustained population growth. Quantitatively, periods of unrest included many times more events of instability per decade and occurred when the population was declining, rather than increasing. Pre-industrial agrarian societies typically faced instability after one or two centuries of stability. However, a population approaching its carrying capacity alone is not enough to trigger general decline if the people remained united and the ruling class strong. Other factors had to be involved, such as having more aspirants for positions of the elite than the society could realistically support (elite overproduction), which led to social strife, and chronic inflation, which caused incomes to fall and threatened the fiscal health of the state. In particular, an excess in especially young adult male population predictably led to social unrest and violence, as the third and higher-order parity sons had trouble realizing their economic desires and became more open to extreme ideas and actions. Adults in their 20s are especially prone to radicalization. Most historical periods of social unrest lacking in external triggers, such as natural calamities, and most genocides can be readily explained as a result of a built-up youth bulge. As those trends intensified, they jeopardized the social fabric, which facilitated the decline."