"This paper seeks an explanation for the well-documented wage disadvantage of mothers compared to women without children. An analysis of data from the 1968-88 National Longitudinal Survey of Young Women shows that human capital inputs and unobserved heterogeneity explain 55-57% of the gap. Further analysis suggests that mothers tended to face the highest wage penalty when they first returned to work. A finding that medium-skill mothers (high school graduates) suffered more prolonged and severe wage losses than either low- or high-skill mothers casts doubt on the work-effort explanation for the wage gap, according to which women reduce work effort in response to childcare duties. The authors instead cite variable time constraints: high school graduates are likely to hold jobs requiring their presence during regular office hours, and are unlikely to gain flexibility by finding work at other hours or by taking work home in the evening."

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