The New Palgrave Dictionary of Economics Online

A Laffer curve is a hump-shaped curve showing tax revenue as a function of the tax rate. Revenue initially increases with the tax rate but then can decrease if taxpayers reduce market labour supply and investments, switch compensation into non-taxable forms, and engage in tax evasion. The revenue-maximizing tax rate can be calculated from an estimate of the elasticity of taxable income with respect to the after-tax share. Some studies find this elasticity to be near zero, and others find it to exceed 1. The mid-range for this elasticity is around 0.4, with a revenue peak around 70 per cent.