Top incomes in the Netherlands and the United Kingdom over the 20th Century

A method is developed for using income-tax data to investigate the evolution of the highest incomes over virtually the entire 20th century. The income shares of the top 10, 5, 1, 0.5, 0.1, and 0.05 percent are analysed for the UK and the Netherlands. For considering the top shares among themselves the "Pareto–Lorenz coefficient" is proposed. Between the two countries, the top shares appear to undergo a strikingly similar and strong decline up to the mid-1970s. Since then British top shares have increased significantly while Dutch shares remained basically unchanged. This outcome parallels similar results for the US and France obtained by Piketty and Saez and poses interesting questions for research.