You learn the value from your family. Families which transmit this value have a powerful natural-selection advantage, so you're more likely to come from a fami

You learn the value from your family. Families which transmit this value have a powerful natural-selection advantage, so you're more likely to come from a family which does so. There's also a powerful advantage to discouraging family members from examining the value too closely, so the value tends to be held in some kind of religious regard, and the idea of rejecting it tends to be seen as selfish, even though bringing a child into the world is the biggest claim on community resources most individuals can ever make.