We know that electoral systems have an effect on the number of competing candidates. However, a mystery remains concerning the impact of majority runoff. According to theory, the number of competing candidates should be equal (or only marginally larger) under majority runoff than under plurality. However, in real-life elections, this number is much higher under majority runoff. To provide new insights on this puzzle, we report the results of a laboratory experiment where subjects play the role of candidates in plurality and majority runoff elections. We use a candidate-only and sincere-voting model to isolate the effect of the electoral system on the decision of candidates to enter the election. We find very little difference between the two electoral systems. We thus re-affirm the mystery of the number of competing candidates under majority runoff.