Humans are not capable of measuring to that level of detail. It should be expected there will be externalities. So far, the only way I have thought of that so

Humans are not capable of measuring to that level of detail. It should be expected there will be externalities. So far, the only way I have thought of that society can align productivity of raising children well to the actual benefit of raising children well is by scrapping all old age benefits. But even that might not work in reality.