The goal of this article is to explain cross-country differences in over- and under-employment. The focus is on the effects of the growth of part-time work. We argue and demonstrate that the spread and acceptance of part-time work results in a downward adaptation of descriptive norms regulating work hours. Initially, women use part-time work to solve the Work-Family-Conflict. The increase of part-time work among women with children increases its acceptance among women and men, as part-time work offers the opportunity to combine paid work with education, care, or the anticipation of retirement. Following this argument, we deduce hypotheses about the work hour preferences of full-time and part-time working women and men and test these hypotheses on the second wave of the European Social Survey (ESS, 2004). The results show that a larger share of part-time work in a country is associated with stronger preferences among full-time workers to reduce work hours. The preference to reduce work hours spreads among full-time working mothers, and from mothers to other full-time working women and men. Preferences to increase working hours among part-time workers are more driven by the level of prosperity of the country.