A guaranteed minimum income ensures that every citizen will be able to purchase a desired level of basic needs. A basic income (or negative income tax) is a system of social security, that periodically provides each citizen, rich or poor, with a sum of money that is sufficient to live on. Studies of large cash-transfer programs in Ethiopia, Kenya, and Malawi show that the programs can be effective in increasing consumption, schooling, and nutrition, whether they are tied to such conditions or not. Proponents argue that a basic income is more economically efficient than a minimum wage and unemployment benefits, as the minimum wage effectively imposes a high marginal tax on employers, causing losses in efficiency. In 1968, Paul Samuelson, John Kenneth Galbraith and another 1,200 economists signed a document calling for the US Congress to introduce a system of income guarantees. Winners of the Nobel Prize in Economics, with often diverse political convictions, who support a basic income include Herbert A. Simon, Friedrich Hayek, Robert Solow, Milton Friedman, Jan Tinbergen, James Tobin