The M + 1 Rule, that at equilibrium there should be only one more candidate running than seats available, extended Duverger's Law to the cases of more than one seat per district. Both the M + 1 rule and Duverger's Law have been confirmed repeatedly, albeit always with qualification. Yet we have reached no consensus on the mechanism that produces these two empirical regularities. In this paper I use a simple simulation to test the hypothesis that the mechanism is that candidates retire after some fixed number of consecutive losses. I test the results of the simulation against several different empirical results and find a reasonably good fit. I also add a mechanism for increasing the number of candidates. These results suggest that the M + 1 rule and Duverger's law may be the result not of rational calculation but of some simple set of decision rules. The analysis also illustrates the usefulness of a simulation approach to hypothesis testing.