[...] the 1980s, when temporary staffing agencies like Kelly Girl Service and Manpower contracted out more workers than most companies retained as full-time workers. Manpower's temp pool surpased GM's full-time employees in the 1990s. Sociologist Erin Hatton makes a compelling case that the wholesale swapping out of full-time employees for contingent staff was not just the fallout of technological change. She charts the staffing and temp agency sector's growth as its own service industry. What did it sell? The suggestion that young women and others, constrained by family care, limited educational opportunities, or geography, would reduce the companies' costs. Hatton argues that this "liability model," which treats workers as a drag on corporate profits, overtook the "asset model" of employment. [...]