In the past, the impact of high marginal tax rates lower down the scale was dismissed on the grounds that many people had little discretion about their working hours or intensity of work. However, even if that were true in the past, it has become less so in a labour market where there is greater fluidity. [...]
for someone working (say) forty hours a week, high marginal tax rates are created by the withdrawing of income-tested benefits (entitlement to tax credits), so they can face marginal tax rates of ~70%