Perhaps much larger birth subsidies would help to raise birth rates. But the world with much larger birth subsidies might also coincide with a world where the social pressures to have (more) children are higher than under the status quo. Many women would be worse off with such pressures, or at least they feel they would be. In principle, these women might prefer a mixed regime of “strong pecuniary incentives, weak social incentives,” but they don’t feel they can get to that mix. The expressive function of the law is strong, and the social pressures to have more children would become strong too. These women therefore end up opposed or indifferent to the pecuniary incentives, even though on paper they might appear to be a “free lunch.”
The difficulty of separating pecuniary and social incentives therefore may be one reason why TFR problems are hard to fix.