This essay uses the labour theory of value as interpreted by Duncan Foley (and Gérard Duménil) to examine the long run development of the US rate of profit since 1890. In particular, the crises beginning in 1929, 1979 and 2007 are compared, and related to trends in the rate of profit. The essay explores the consequences of considering aggregate profits not as total money value added less total wages, but as total money value added less total working class wages, and finds trends in the rate of profit, so redefined, to be more closely aligned with crisis in 1929 and 2007 than the conventionally defined rate of profit.