In many occupations workers’ labor supply choices are constrained by institutionalnrules regulating labor time and effort provision. This renders explicit tests of the neoclassicalntheory of labor supply difficult. Here we present evidence from studies examining labornsupply responses in “neoclassical environments” in which workers are free to choose whennand how much to work. Despite the favorable environment the results cast doubt on thenneoclassical model. They are, however, consistent with a model of reference dependentnpreferences exhibiting loss aversion and diminishing sensitivity.