[Excerpt] In accord with the relatively strong role that market forces have played in American economy history, the United States has long been noted for a high degree of diversity in the conditions under which employees work. Yet in recent years the amount of labour market diversity has increased markedly, spurred in part by the share of the labour force represented by unions continuing to decline (from a peak of 35 per cent in the early 1950s to 20 per cent in 1983 and to 12 per cent in the early twenty-first century) (Bureau of Labor Statistics 2008a).