Over the years 2001–2013, the share of individuals in the United States paying positive income taxes fell 6 percentage points, while the fraction with zero and negative tax liabilities increased by 1 and 5 percentage points, respectively. However, during 2008 through 2010, the fraction with negative tax liabilities increased substantially, with corresponding declines in positive and zero tax liabilities. In this paper, we examine the extent to which changes in income tax law, as compared to changes in characteristics of the population represented on income tax and information returns, drove these changes. For this study, we use a sample of tax and information returns drawn from the population of U.S. individual income tax returns over the years 2001–2013. Using this data, combined with federal income tax calculators, we first simulate counterfactual trends in taxpaying status (whether tax liability is positive, zero, or negative) assuming constant tax policy and the actual year-specific populations. We then calculate counterfactual trends in taxpaying status assuming a constant population but actual year-specific tax policy. The results suggest that changes in the population drove most of the overall trends in taxpaying status over the sample period, but changes in tax policy drove almost all of the sizable drops in positive and zero tax liabilities and increase in negative tax liabilities in 2008 through 2010.