It was in the interests of parents in the agricultural era to have as many children as possible so that they could be looked after in their old age by enough children who survived diseases and the general mishaps of life.
In the early half of the industrial age, with better disease control and a cornucopia of consumer goods becoming available, parents could afford to have far fewer children — down to somewhat above two children per woman, the replenishment rate — in order to be looked after in old age and continuing to enjoy their consumer luxuries.
In the later half of the industrial age, parents with private pension funds and the state apparently providing for their old age, and affording the full kit of status goods that was available and a satisfying range of leisure activities in their increasing years of retirement from work, decided to have far fewer children than replacement rate, even though by this strategy they ought to have been aware that the population was heading for extinction in due course.
In the early half of the post-industrial age, the supposedly ‘elite’ part of the population — 15% to 20% — realising that they would probably not be able to continue to subsidise the rapidly increasing welfare demands of the rest of the population, have generally decided to become culturally independent. At the same time, they are now deciding that they might as well start to recover the joys of having replenishment-size families and are now reversing the extinction trends of the remainder of the population.
Thus, in our increasingly competitive world, our instinctive social pecking order is being forced to increasingly grim extents both within nation-states and between them.