We saw in Chapter 10 that in Costa Rica, the rule requiring a candidate to obtain at least 40% of the vote or else face a runoff has led to a pattern of competition almost identical to that seen in straight plurality presidential systems. The Costa Rican rule thus contrasts rather starkly with majority provisions, in which there is a runoff in the event that no candidate obtains at least 50% plus one of the votes. In such majority runoff systems, the vote shares of the first and second finishers in the first round tend to be considerably lower than in plurality rules, including the Costa Rican variant.

Is this outcome fortuitous, or is there something to the 40% rule that encourages broad preelection coalitions as does pure plurality? This question is worth investigating, especially given that there is only one case of a system using the 40% rule. Perhaps Costa Rica, where a runoff has never been required, is a special case because of its single well-organized mass party, the Party of National Liberation (PLN). Perhaps this party has a “natural” share of the vote that approximates 50% anyway. If so, it might ordinarily win elections (which it certainly does), and it could only lose to a united opposition – that is, one that could itself approach the magical 50% mark. Indeed, in six of the ten elections since 1958, the PLN has obtained an absolute majority.