> Perhaps I'm missing something, but how does the human body have internal knowledge of the current gender ratio in society such that it will attempt to fix the ratio to balance?
It doesn't.
This process happens strictly through natural selection.
If average female fertility is F, the number of sexually mature females is f and the number of sexually mature males is m, then the average male fertility is F * f / m (each child is conceived by exactly one male and one female). This means that the less numerous sex has a higher fertility.
It is not true that each child is equally likely to be male or female. Some parents are more predisposed to have male children and some parents are more predisposed to have female children. The parents predisposed to have more reproductively successful children will have their genes flourish - pushing the sex balance of new babies slightly in the corrective direction.
It doesn't.
This process happens strictly through natural selection.
If average female fertility is F, the number of sexually mature females is f and the number of sexually mature males is m, then the average male fertility is F * f / m (each child is conceived by exactly one male and one female). This means that the less numerous sex has a higher fertility.
It is not true that each child is equally likely to be male or female. Some parents are more predisposed to have male children and some parents are more predisposed to have female children. The parents predisposed to have more reproductively successful children will have their genes flourish - pushing the sex balance of new babies slightly in the corrective direction.