• globalisation
• technological change (information and communications technology)
• growth of financial services
• changing pay norms
• reduced role of trade unions
• scaling back of the redistributive tax-and-transfer policy

[...] we risk creating the impression that inequality is rising on account of forces outside our control. It is far from obvious that these factors are beyond our influence or that they are exogenous to the economic and social system. Globalisation is the result of decisions taken by international organisations, by national governments, by corporations, and by individuals as workers and consumers. The direction of technological change is the product of decisions by firms, researchers, and governments. The financial sector may have grown to meet the demands of an aeing population in need of financial instruments that provide for retirement, but the form it has taken and the regulation of the industry have been subject to political and economic choices.

In the case of globalisation, what lies behind is the standard international trade model (usually referred to as the Heckscher–Ohlin model after the two Swedish economists Eli Heckscher and Bertil Ohlin), according to which the classes of workers, skilled and unskilled, are employed in two perfectly competitive economies, each with two sectors of production.

[...] the decline in unionisation is the result of the bias in technical change towards skilled workers. Technological change biased towards skilled workers undermines the coalition between them and unskilled workers that provides the basis for union bargaining power, and the consequent decline in unionisation amplifies the rise in wage dispersion.