People don’t have children due to some particular genetic predisposition (every human has this), they do so for various reasons that involve the psyche which in

People don’t have children due to some particular genetic predisposition (every human has this), they do so for various reasons that involve the psyche which influence the will and the psyche is heavily influenced by social conditions. This is to say that even if someone comes from a large family they are going to be influenced by what they see on TV, learn in school, etc and even more broadly, nowadays there is no survival necessity to have children, they are not needed to work the family farm or provide elder care. People have children out of love and desire to bestow goodness to someone else besides them. It takes a lot to raise someone to have these selfless qualities and virtually no society has succeeded in this.