In 2002, Sweden reformed its parental leave system by adding a second “daddy month,” i.e., a second month of pay-related parental leave reserved exclusively for each parent. In addition to giving fathers an economic incentive to take more leave, this change had an effect on cultural norms. We develop and estimate a model of the household in which preferences towards leave depend on the behavior of one’s peers and use it to quantify the magnitudes of the economic-incentive effects as well as the evolving norms. We find that endogenously evolving cultural norms play a major role. We use our model to evaluate the effects of several potential policy changes including decreasing the cost of child care and giving each parent a substantially larger non-transferable endowment of parental leave and conclude that only the latter would have a significant effect on the share of parental leave taken by men.