Abstract: Mexico has experimented with mixed-member electoral systems for many years, is probably the second oldest mixed-member system after Germany, and has modified its mixed system more than any other country. The purpose behind the electoral reforms has always been the same: to open up the system enough to satisfy political demands, but not so much that the hegemonic party loses control. Although the latter could to a large extent be controlled by electoral fraud, the former objective still had to be addressed, and over the last 35 years Mexico has seen a gradual but consistent expansion of proportional representation, so that by 1997, the electoral laws, together with electoral preferences and the general democratization of the country, had led to the end of majority control by the official party in the Chamber of Deputies, and Mexico now has a strong two- or three-party system. However, there have been seven fundamentally different electoral systems in Mexico this century, with a different electoral system for each of the last five elections, which makes it impossible to distinguish the effects of electoral rules from those of voter preference; furthermore, most of the reforms are highly endogenous with the party system. Discusses reforms from 1964 onwards, and is arranged as follows: it first describes the party (minority) deputy system that operated under single-seat district plurality rules in the period 1964–1976, and then the minority representation system of 1979–1985 (mixed-member majoritarian (MMM) rules were introduced for the 1979 election); next it gives accounts of the 1988, 1991, 1994 and 1997 mixed-member electoral laws, and details of the senate formulas for the period 1994–2000; it then discusses the consequences of the four mixed-member electoral laws (effects on party competition and Duvergerian effects), the effects of the change to an MMM system on legislative behavior, and the prospects for future electoral reform.

Keywords: Duvergerian effects, electoral reform, electoral rules, electoral systems, legislative behavior, Mexico, minority deputy system, minority representation system, mixed-member electoral systems, mixed-member majoritarian systems, party competition, plurality rules, proportional representation,