together with income taxes and social security contributions, were estimated to reduce inequality among the working-age population (measured by the Gini coefficient) by an average of about onequarter across OECD countries during the period from the mid-1980s to the late 2000s.122 Of course, as noted above, a progressive tax system only can have an impact on the reduction of inequalities if the revenue from the taxes collected is redistributed through social policies that benefit the poor, rather than being spent on investments that shall only allow the rich to become richer. But, both by reducing the level of pre-tax income inequalities and by increasing the fiscal capacity of the State, a progressive tax system has an important role to play in the fulfilment of social rights. The Committee on Economic, Social and Cultural Rights has therefore regularly expressed its concern at reforms of the taxation system that would make it less progressive (for instance, by shifting the burden from corporations to the families, or by increasing VAT rates on essentiel items): in the most recent Concluding Observations addressed to the United Kingdom, it deplored "the adverse impact that recent changes to the fiscal policy in the State party, such as the increase in the threshold for the payment of inheritance tax and the increase of the value added tax, as well as the gradual reduction of the tax on corporate incomes, are having on the ability of the State party to address persistent social inequality and to collect sufficient resources to achieve the full realization of economic, social and cultural rights for the benefit of disadvantaged and marginalized individuals and groups".123 Two remarks are in order, however. First, the ability for even a progressive tax system to reduce inequalities depends not only on the contribution of the richest part of the population to public revenue in percentage terms (as a proportion of the total tax burden), but also on the absolute levels of such contributions : if, for example, the richest decile of the population pays 90 per cent of the total income taxes collected in the country, the taxation system may be said to be progressive according to the most common measure of tax progressivity known as the Kakwani index. But if those richest 10 per cent are taxed at very low rates, the redistributive capacity of the taxation remains very limited: such a redistributive capacity is captured by another index, known as the Reynolds-Smolensky index, which measures the difference in income distribution before and after the tax is imposed.124 One important consequence of this distinction is that a tax reform that may at first appear as regressive because the proportion of the total tax revenue paid by the richest part of the population will decrease (if the reform extends the tax base to ensure that the effort is spread across a larger part of the population), nevertheless may have progressive consequences if the overall tax rates and thus the revenue capacity of the State are increased. This again provides an illustration of the need to consider revenue mobilization and spending together, as they relate to one another, rather than to treat them separately. The second remark concerns the relationship between higher levels of taxation and the introduction of more progressive systems of taxation, on the one hand, and wealth creation, on the other hand. Contrary to a widely held assumption,125 high levels of taxation, allowing the State to adopt robust redistributive policies and provide high-quality public services, are not an obstacle to economic 122 Organisation for Economic Co-operation and Development, Divided we Stand: Why Inequality Keeps Rising (OECD, Paris, 2011). 123 Committee on Economic, Social and Cultural Rights, Concluding Observations: the United Kingdom of Great Britain and Northern Ireland, E/C.12/GBR/CO/6 (14 July 2016), para. 16. 124 The Kakwani and the Reynolds-Smolensky indexes appeared simultaneously in the economic literature : see Nanak C. Kakwani, "Measurement of Tax Progressivity: An International Comparison", The Economic Journal, vol. 87 (345) (1977): 71–80; and Morgan O. Reynolds and Eugene Smolensky, Public Expenditures, Taxes and the Distribution of Income: The United States, 1950, 1961, 1970 (New York, Academic Press, 1977). For a general presentation, see Jonathan Haughton and Shahidur Khandker, Handbook on Inequality and Poverty (Washington, D.C.: World Bank, 2009) (chap. 15: "The Effects of Taxation and Spending on Inequality and Poverty"). The reliance on these measures has been criticized on the ground that they fail to take into account the changes in revenue that may result from the introduction of tax reforms: see Santiago Díaz de Sarralde, Carlos Garcimartín and Jesús Ruiz-Huerta, "The paradox of progressivity in low-tax countries: income tax in Guatemala", CEPAL Review, n° 102 (Dec. 2010), pp. 85-99. 125 The assumption was popularized as the "Kuznets curve" after the work of Simon Kuznets who hypothesized in the mid 1950s that inequality was an inevitable price to pay during industrialization in fast-growing nations (see Simon Kuznets, "Economic Growth and Income Inequality", American Economic Review 45 (March 1955): 1–28). However, quite apart from the fact that such a reasoning could not be transposed to advanced industrial economies in which the processes of urbanization and structural transformation associated with industrialization are completed, the ideological uses made of his work do not correspond to the actual findings of Kuznets; nor do such uses have any (other) solid data to rely on. 28