We investigate heterogeneity of wages and wage growth rates across different working hours to quantify the implications of splitting work equally among spouses. Based on a combination of German administrative and survey data, we account for endogenous selection into specific hours by exploiting reforms of the tax system. We find substantial heterogeneity in part-time wage penalties, ranging from -2% to -18% compared to full-time. The heterogeneity in wage growth penalties is similar, but less severe. Both penalties are not linearly decreasing in working hours. High penalties for working high part-time hours suggest that splitting work equally would imply sizeable wage losses.