The idea of positive externalities (also called "external benefits") is definitely mainstream economics. Deciding a priori what the "right" population level for

The idea of positive externalities (also called "external benefits") is definitely mainstream economics. Deciding a priori what the "right" population level for the United States is seems philosophical to me. There are plenty of other rebuttals to the argument I gave, though. For example: 1) Negative externalities from having children (crowding, etc.); 2) Deadweight losses caused by the taxes used to pay for the subsidy; 3) Calculation problems: deciding the proportion of internalized vs socialized benefits from children seems almost impossible to calculate even on average; if you get that wrong, your subsidy could easily do more harm than good; 4) Government power: even if this particular subsidy is efficient, if you give your government the authority to tax and subsidize broadly, they are likely to make plenty of bad decisions, which could partially or totally offset the benefits