This work provides a parametric and semi-parametric analysis of the relationship between the proportionality degree of an electoral system and corruption. This allows us to properly consider mixed electoral systems alongside the two traditional ones, proportional and plurality. Results show that a reduction in the proportionality degree within the same proportional system is not beneficial in fighting corruption because it weakens the monitoring power of opponents (their representativeness reduces) without the introduction of the voters’ monitoring. On the contrary, mixed rules allow both monitors to exercise their power to induce politicians to avoid corrupt behaviour. Increasing plurality elements into mixed systems is beneficial only up to certain proportionality degrees, after which the corresponding level of corruption begins to grow. Therefore, for governors who want to adopt mixed electoral systems, the choice of their proportionality degree becomes fundamental.