Super PACs, since their inception in 2010, have played a significant role in federal campaign finance. In 2016 alone, super PACs spent over $1 billion trying to influence elections. In particularly close contests with significant disparities in independent expenditures (the main tool of super PACs), this money can play an important role in the outcome. Despite the prominent role of super PAC money in federal elections, limited research has been conducted on the sources of this money. While speculation about mega-donors, corporations, unions, nonprofits, and shell companies has dominated the news media and political punditry, these sources of funds have yet to receive a rigorous quantitative treatment. Individual contributions to super PACs,...