> In the past, quality of life was terrible compared to modern life but fertility was not an issue.

not sure which "past" you're referring to, but in agricultural societies, more kids was important to survival and quality of life, as you needed hands on the farm; also, the child mortality rate was much higher so you had to have more kids to start with; that was also pre-birth control -- as soon as that was introduced the birth rate started to fall tremendously