"[146]
Mellon ordered a study that demonstrated historically that, as income tax rates were increased, money was driven underground or abroad, and he concluded that lower rates would increase tax revenues.[147][148] Based on his advice, Harding's revenue bill cut taxes, starting in 1922. The top marginal rate was reduced annually in four stages from 73% in 1921 to 25% in 1925. Taxes were cut for lower incomes starting in 1923, and the lower rates substantially increased the money flowing to the treasury. They also pushed massive deregulation, and federal spending as a share of GDP fell from 6.5% to 3.5%. By late 1922, the economy began to turn around. Unemployment was pared from its 1921 high of 12% to an average of 3.3% for the remainder of the decade. The misery index, a combined measure of unemployment and inflation, had its sharpest decline in U.S. history under Harding. Wages, profits, and productivity all made substantial gains; annual GDP increases averaged at over 5% during the 1920s. Libertarian historians Larry Schweikart and Michael Allen argue that, "Mellon's tax policies set the stage for the most amazing growth yet seen in America's already impressive economy.