A selective brake uses government's inherent capacity to slow economic activity deliberately on the parts of the economy we want less of — pollution, overfishing, systemic financial risk — rather than applying it indiscriminately across the whole economy. The problem is that most real-world government spending (wages, transfers, subsidies, procurement) is not a targeted instrument: it applies the same braking force to productive and unproductive activity alike. The aggregate data captures this reality — countries with large governments are not primarily running externality-correction programs; they are running redistribution and public employment programs.