>Randomly assign a representative sample of the population - say, 10,000 taxpayers - a lower tax rate, and see what happens.
This assumes the tax rate only affects the individual paying tax; but taxes are used at least in part to finance public goods - positive externalities that also affect individuals. If a random group of people personally pay lower taxes but still benefit from the available public services that other taxpayers are still funding, that doesn't tell us anything useful.
This assumes the tax rate only affects the individual paying tax; but taxes are used at least in part to finance public goods - positive externalities that also affect individuals. If a random group of people personally pay lower taxes but still benefit from the available public services that other taxpayers are still funding, that doesn't tell us anything useful.