The single largest factor in shaping world demography has been the global decline in childhood death rates.
Throughout human history around half of humans died during childhood, and without reproducing:
In developed countries, almost all children (about 99 percent) now survive to adulthood, and even among the most impoverished, ignorant or undeveloped segments of these populations, the proportion of children who die during childhood is biologically almost insignificant.
The above 50 percent childhood death rate is only an average - among the poorest sector of the population the childhood mortality probably approached fairly close to 100 percent.
So it did not matter much how many children were born to the poor, since almost none of them would survive - and extra children born to the poorest and most ignorant classes probably served merely to reduce the survivorship of those children who had not reached adulthood.
But childhood mortality was probably considerably lower than 50 percent among the wealthier, more intelligent, higher in status.
Therefore, the modern population in developed countries (you and I) are almost entirely the offspring of the wealtheir, more intelligent, more conscientious, higher status classes of history.
So, in the past, it was childhood death rates which - mostly - drove demographic change. Things that reduced childhood death rates (more food, less disease, less violence, fewer fatal accidents, better hygeine, better medicine, better - more loving, intelligent and conscientious - mothers) would increase reproductive success.
But in the modern world nowadays reproductive success is essentially a matter of birth rates: of fertility. Childhood mortality is so low it can (almost) be ignored (I mean ignored from a strictly biological point of view).
No matter how relatively underdeveloped and underprivileged a population - no matter how stupid, feckless and uncaring the mothers - for almost everywhere in the world at present, childhood death rates are all-but irrelevant to reproductive success: almost all children are 'kept alive' by 'society'.
Consequently, for the first and only time in human history, pure fertility drives demographic change - and also evolutionary change.