Third, women might forego higher-paying employment in favour of family-friendly jobs that allow them to work fewer hours and spend more time at home. On the other hand, discrimination explanations are based on the notion that even though mothers may be equally productive, employers may (for strategic reasons) pay them less than non-mothers and men of similar productivity in the labour market. Using longitudinal surveys from the United States, Waldfogel (1997) concludes that an unexplained motherhood wage penalty persists after controlling for human capital, unobserved heterogeneity and part-time employment, with a 4% penalty for one child and a 12% penalty for two or more children.