Move from the small town to the city, and children quickly (d)evolved (in economic terms) into being little more than really pricy conversation pieces. And while more than one parent cries tears of sad joy when the kids finally move out, there tends to be little of the panic that would have occurred had such vacating happened on a preindustrial, near-subsistence-level farm. When much of the economic rationale for having children evaporates, people do what comes naturally: they have fewer of them.
