When growth gives way to decline, population typically keeps rising for another generation or so due to sheer demographic momentum, and then begins to fall. The consequences can be traced in the history of every collapsing civilization. As the rural economy implodes due to agricultural failure on top of the more general economic decline, a growing fraction of the population concentrates in urban slum districts, and as public health measures collapse, these turn into incubators for infectious disease. Epidemics are thus a common feature in the history of declining civilizations, and of course, war and famine are also significant factors: but an even larger toll is taken by the constant upward pressure exerted on death rates by poverty, malnutrition, crowding, and stress. As deaths outnumber births, population goes into a decline that can easily continue for centuries.
This scenario is the standard picture of a dark age society;
a mostly deserted countryside of small and scattered villages where subsistence farmers, illiterate and impoverished, struggle to coax fertility back into the depleted topsoil. Their governments consist of the personal rule of local warlords, who take a share of each year’s harvest in exchange for protection from raiders and rough justice administered in the shade of any convenient tree. Their literature consists of poems, lovingly memorized and changed to the sound of a simple stringed instrument, recalling the great deeds of the charismatic leaders of a vanished age, and these same poems also contain everything they know about their history. Their health care consists of herbs, a little rough surgery, and incantations cannily used to exploit the placebo effect.
— John Michael Greer, Dark Age America