Multi-member districts employing a one-vote-per-member system appear to reach an equilibrium consistent with the M+1 rule. This rule sates that the number of vote-receiving candidates in a district is limited to the number of seats in a district (M) plus one. Additional candidates are unlikely to have on impact on the election if the district is in equilibrium. The M+1 hypothesis is tested using data from Japan and results and their implications are discussed.