The Petrobras scandal currently engulfing Brazil is unprecedented in its scale and scope. Ironically, when the party of President Dilma Rousseff, the Workers’ Party (PT), initially became a major political player in 1989, it was seen as a clean, ethical alternative after President Collor de Mello stepped down from office amidst corruption allegations. Yet in the years following its rise to power, the PT has been dogged by corruption allegations, even before the explosion of the Petrobras investigations. During the presidency of Ms. Rousseff’s predecessor, Lula Inácio da Silva, prosecutors unearthed a major scheme, known as the Mensalão scandal, under which public funds were being used to pay members of Congress in exchange for their support of the PT government in crucial votes. At the end of the inquiry, 25 politicians and businessmen were convicted. Several other smaller corruption schemes (including Caixa Dois, Bingos, Sanguessugas, and Dossier) also implicated high-ranking members of the PT during Mr. da Silva’s tenure.

Despite this clear evidence of corruption within the PT ranks, the PT has been able to maintain its relative dominance in Brazilian politics, with three successive victories in presidential elections following Mr. da Silva’s initial rise to power, including Ms. Rousseff’s re-election in 2014, six months after the launch of the Petrobras investigations. This raises yet again a question that scholars and commentators have asked over and over again: Why do voters keep re-electing corrupt politicians? Democracy is supposed to enable voters to hold their government accountable, and most voters claim to dislike corruption and to value integrity in government. So why do parties like the PT keep winning elections? While there are many possible explanations (maybe, for example, voters don’t really care as much about corruption as they claim), part of the explanation in certain countries may have to do with the particularities of the electoral system.

Brazil has a hybrid electoral system: the President is elected in a two-round majority run-off system, elections for the Senate are based on plurality votes within states, and elections to the Chamber of Deputies are based on open-list proportional representation. An examination of this system suggests that it is particularly inimical to holding corrupt politicians accountable, and may have in fact contributed to the seemingly intractable problem of corruption in Brazilian politics. Three problems in particular stand out:

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