New Delhi, June 22 -- The surest way for policy advocates to lose a progressive audience is to talk about the economy's supply side, the importance of incentives and the dangers of overregulation-ideas typically linked to conservative agendas. Ezra Klein and Derek Thompson's new bookAbundanceaims to change all that.

As the authors point out, the left has traditionally focused on demand-side remedies. A key tenet of the New Deal in the US and social democracy in Europe is Keynesian management of aggregate demand to ensure full employment.

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Klein and Thompson rightly underscore that it is improvements in supply that are the source of broad-based posterity in th...