from the Richmond Fed
More than half of the U.S. population is covered by health insurance received through employment. Employer-tied health insurance grew quickly after key policy and tax changes in the 1940s and 1950s, but critics argue that the system distorts incentives and has contributed to rising health care costs.
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Source: http://www.richmondfed.org/publications/research/econ_focus/2014/q1/pdf/economic_history.pdf