同龄人的影响在参加社会项目方面可以发挥重要作用。然而，考虑到反射、相关不可观测物和内生群体成员等问题，估计同伴效应具有挑战性。我们在挪威带薪陪产假的背景下使用回归间断设计克服了这些识别问题。我们的方法不同于现有的文献，这些文献试图通过对同伴群体进行随机分配来衡量同伴效应;相比之下，我们研究自然发生的同伴群体中的同伴效应，但利用社会计划对个体子集的“价格”的随机变化。在挪威，1993年4月1日之后出生的孩子的父亲有资格享受一个月的政府带薪陪产假，而在此截止日期之前出生的孩子的父亲则没有。改革后，立即休陪产假的父亲人数急剧增加，从3%上升到35%。虽然这种准随机变化改变了一些父亲的陪产假成本，但它没有直接影响父亲的同事或兄弟的成本。因此，对兄弟或同事的任何影响都可以归因于同伴父亲在他们网络中的影响。我们关于同伴效应的主要发现有四个方面。 First, we find strong evidence for substantial peer effects of program participation in both workplace and family networks. Coworkers and brothers are 11 and 15 percentage points, respectively, more likely to take paternity leave if their peer father was induced to take up leave by the reform. Second, the most likely mechanism is information transmission about costs and benefits, including increased knowledge of how an employer will react. Third, there is essential heterogeneity in the size of the peer effect depending on the strength of ties between peers, highlighting the importance of duration, intensity, and frequency of social interactions. Fourth, the estimated peer effect gets amplified over time, with each subsequent birth exhibiting a snowball effect as the original peer father's influence cascades through a firm. Our findings demonstrate that peer effects can lead to long-run equilibrium participation rates which are substantially higher than would otherwise be expected.