It depends not just on absolute level of ease but relative. Suppose having and raising a child takes some normalized 100 units of work and some basket of luxuri

It depends not just on absolute level of ease but relative. Suppose having and raising a child takes some normalized 100 units of work and some basket of luxuries takes the same 100 units. Even if having a child drops to 50 units of work, if that same basket of luxuries drops to 25 units, you'll still see a decrease in fertility.