predicts that the share of the workforce employed in low-productivity industries will rise over time. The reason for this can be seen through a thought experiment offered by Baumol in his book The Cost Disease: Let us assume for simplicity that the share of the economy's total output that comes from the progressive sector [industries with high productivity growth], as measured in physical units rather than money, does not change. Because the economy has only two sectors, progressive and stagnant [industries with low productivity growth], whose production together accounts for all of its output, it follows that the stagnant sector also must maintain a constant share of the total. This has significant implications for the distribution of the economy's labor force. By definition, labor productivity grows significantly faster in the progressive sector than in the stagnant sector, so to keep a constant proportion between the two sectors' output, more and more labor has to move from the progressive sector into the stagnant sector. As predicted by the Baumol effect, the proportion of the United States labor force employed in stagnant industries has grown substantially since the 1960s. In particular, the United States has morphed from a manufacturing economy into a service economy (see chart). However, how much of this is due to the Baumol effect rather than other causes is disputed. In a 2010 study, the economist Talan B. İşcan devised a model from which he concluded that both Baumol and Engel effects played significant roles in the rising share of employment in services in the United States (though he noted that "considerable gaps between the calibrated model and the actual data remain"). An older 1968 study by economist Victor Fuchs likewise concluded that the Baumol effect played a major role in the shift to services, although he determined that demand shifts like those proposed in Engel's law played only a minor role. The economists Robert Rowthorn and Ramana Ramaswamy also concluded that relatively faster growth of productivity in manufacturing played a role in the shift to services. The economist Tom Elfring, however, argued in a 1989 paper that the Baumol effect has played a secondary role to growth in demand for services since the 1970s. Alternative theories for the shift to services include demand-side theories (the Baumol effect is broadly a supply-side explanation) like the three-sector model devised by Allan Fisher and Colin Clark in the 1930s, which posit