A search of recent political science literature and conference presentations shows substantial fascination with the concept of the natural experiment. However, there seems to be a wide array of definitions and applications employed in research that purports to analyze natural experiments. In this introductory essay to the special issue, we attempt to define natural experiments and discuss related issues of research design. In addition, we briefly explore the basic methodological issues around the appropriate analysis of natural experiments and give an overview of different techniques. The overarching theme of this essay and of this issue is to encourage applied researchers to look for natural experiments in their own work and to think more systematically about research design.

We report the results of a field experiment conducted in New York City during the 2013 election cycle, examining the impact of nonpartisan messages on donations from small contributors. Using information from voter registration and campaign finance records, we built a forecasting model to identify voters with an above-average probability of donating. A random sample of these voters received one of four messages asking them to donate to a candidate of their choice. Half of these treatments reminded voters that New York City's campaign finance program matches small donations with public funds. Candidates’ financial disclosures to the city's Campaign Finance Board reveal that only the message mentioning policy (in generic terms) increased donations. Surprisingly, reminding voters that matching funds multiplied the value of their contribution had no effect. Our experiment sheds light on the motivations of donors and represents the first attempt to assess nonpartisan appeals to contribute.

The Supreme Court's sweeping decision in December, 2003, in McConnell v. FEC was as unexpected as it was dramatic. For months the conventional wisdom had been that the Court was certain to overturn one and possibly both of the main parts of the McCain-Feingold law, or otherwise substantially narrow the statue. Instead, the justices, by a slim 5-4 majority, did just the opposite; not only upholding the law but doing so in terms that virtually echoed the defendants' arguments. Proponents were stunned and thrilled. Opponents were appalled; Justice Scalia called it the “opening act of a national tragedy.” Both sides agreed, however, that the decision in McConnell stands as one of the most important campaign finance and First Amendment precedents in several decades.

In some respects, our work in McConnell v. FEC began years before the passage of McCain-Feingold. During 1996–1997 we served on a commission organized by Herbert Alexander to create a political scientists' view of campaign financing. The majority voted 6 to 3 to recommend removing limits on the amounts that parties could contribute to candidates. Our dissent, with Janet Box-Steffensmeier, asserted that such a move would do little to strengthen parties or make elections more competitive, while increasing the potential for donors to corrupt parties (Citizens' Research Foundation 1997). Shortly afterward, we were contacted by lawyers from the FEC looking for expert witnesses in Colorado Republican II to argue on behalf of congressional limits on parties' “coordinated expenditures” (Sorauf and Krasno 1997). A few years later, we were asked by lawyers from the office of Missouri's Attorney General to testify on behalf of that state's regulation of party finances (modeled on FECA) in Missouri Shrink PAC v. Lamb (Krasno and Sorauf 2000). In all of these cases our “testimony” consisted of lengthy reports written for district courts or panels.