Richard Wilkinson and Kate Pickett, The Spirit Level: Why Greater Equality Makes Societies Stronger (London: Bloomsbury Press, 2009), 75.
472
Haskins and Sawhill, 101.
473
James Heckman and Dimitriy V. Masterov, «The Productivity Argument for Investing in Young Children,» Invest in Kids Working Group, Committee for Economic Development, Working Paper 5 (October 4, 2004): 3.
