Christian Challú, Enrique Seira, and Alberto Simpser
Virtually all theoretical work on elections and collective choice assumes that votes are either tallied perfectly, or that deviations from the truth stem from partisan fraud. However, in large electorates where votes are tallied by hand—as in many modern democracies—the accuracy of vote totals cannot be guaranteed even in the absence of fraud. We provide the first evidence that nonpartisan vote counting mistakes in modern elections are substantial and constitute a threat to electoral outcomes. Using data for the universe of over seventy million voters in Mexico in five national elections, we exploit various procedural randomizations and rule-based discontinuities to show that the fear of violence, fatigue, arithmetic difficulty in the tallying, and the educational attainment of poll booth officials are major drivers of nonpartisan tallying mistakes. Using simulation, we show that even nonpartisan tallying mistakes can alter the results of closely contested races—those most likely to attract controversy in the first place. Our findings highlight an unrecognized challenge to the practice of democracy and collective choice.