Income tax in the U.S. has become less progressive since the late 1970s in spite of rising income inequality. Why? Modeling policy makers as a Ramsey government that may weight heterogeneous households differently, I find that economic changes can explain about 61% of the reduction in progressivity observed. Aging population and declining gender gap induce a less progressive income tax, whereas changing idiosyncratic risks and the declines of labor share and interest rate have the opposite effects. Rising skill premium is about neutral in this regard. The remaining reduction in progressivity implies a shift in the government’s weights towards high-ability households. From a utilitarian point of view, the income tax change since the late 1970s induces a welfare gain equivalent to 2.12% of lifetime consumption.

尽管收入不平等加剧，但自1970年代后期以来，美国的所得税累进税率有所下降。为什么？将决策者建模为拉姆齐政府，可能会对异类家庭施加不同的权重，我发现经济变化可以解释所观察到的累进性下降的61％。人口老龄化和性别差距的缩小导致较低的累进所得税，而特质风险的变化以及劳动份额和利率的下降则产生相反的影响。在这方面，不断提高的技能溢价大约是中性的。进步性的剩余降低意味着政府权重向高能力家庭转移。从功利主义的角度来看，自1970年代末以来的所得税变化带来的福利收益相当于终身消费的2.12％。