Electoral Systems in Latin America:
Explaining the Adoption of
Proportional Representation Systems
During the Twentieth Century
Laura Wills-Otero
ABSTRACT
This article explains the twentieth-century Latin American shift from
majoritarian to proportional representation (PR) electoral systems. It
argues that PR was introduced when the electoral arena changed
significantly and threatened the power of the dominant party. The
adoption of PR was therefore an effort by the established party to
retain partial power in the face of absolute defeat. Majoritarian sys-
tems remained in place when the incumbent party was strong
enough to believe that it could gain a plurality of the votes despite
electoral changes. An empirical analysis of 20 countries over 104
years (1900–2004) provides support for this argument.
I
n 1900, no Latin American country allocated legislative seats with pro-
portional representation (PR), but by the middle of the twentieth cen-
tury, half the region had adopted such an electoral rule, and today all
but three countries still employ this system.
1
What explains the move-
ment from a restrictive majoritarianism to a structure that encourages
political party competition? This study argues that PR—and more
broadly, adjustments to open the electoral system—is introduced when
changes in political conditions threaten the hegemony of the dominant
party. A shift to PR therefore reflects an effort by the established party
to retain partial power when facing the prospect of losing all power
under majoritarian rules.
More specifically, this study proposes that three phenomena
threaten entrenched party dominance: an influx of voters to the elec-
torate; the advent of new political parties that capture a substantial
number of votes from the dominant party; and a shift in the preferences
of the electorate away from the major party. As new or oppositional par-
ties become stronger and newly enfranchised voters from previously
politically marginalized social classes, the middle and working classes,
begin to support these parties, failure to adopt PR would lead to the
absolute defeat of the ruling party; and this prospect generates pressure
on party leaders to strategically adjust the existing electoral system in
the direction of PR.
© 2009 University of Miami

Similar theories have been advanced for industrialized democracies
(Boix 1999; Rokkan 1970).
2
This study expands these theories to the
Latin American context by proposing that the regional adoption of PR
occurred for a reason similar to that of developed countries: a calcula-
tion made by ruling parties perceiving a credible threat to their domi-
nance under majoritarian electoral systems. This argument is elaborated
by suggesting that restrictive majoritarian systems are maintained when
the incumbent party is strong enough to believe that it can gain a plu-
rality of votes despite an increase in the voting population, the emer-
gence of new parties, or a shift in electoral preferences. The theory pre-
sented here also explains adjustments to the degree of electoral system
proportionality, since openings may emerge not only by adopting PR,
but also by increasing the district magnitude (DM) under PR or by
reducing it under majoritarianism.
3
The literature on electoral system
choice has emphasized the shift from one allocation rule to another but
has not considered changes in proportionality. The empirical models for
this study are based on statistical results from an analysis of a new elec-
toral systems dataset of 20 Latin American countries from 1900 to 2004.
4
ELECTORAL SYSTEMS AS A DEPENDENT VARIABLE
The literature on electoral systems is unbalanced, with most attention
focused on the consequences of electoral systems for political stability
and on the number of political parties in both representative institutions
and the electorate (Cox 1997; Duverger 1954; Laakso and Taagepera
1979; Rae 1967; Sartori 1976; Taagepera and Shugart 1989). These stud-
ies have concluded that in addition to the electoral formula, the most
relevant feature is the district magnitude. Indeed, evidence indicates that
“the more legislators per district, the smoother the road for new parties”
(Willey 1998).
5
Fewer studies have addressed the origins of electoral rules, proba-
bly because such electoral modifications are rare and usually occur only
in periods of extraordinary political change. Research for advanced
democracies has devoted attention to the introduction of PR at the
beginning of the twentieth century (Boix 1999; Rokkan 1970), and to the
dynamics of electoral reforms and their impact on party systems. In
Latin America, a relatively large amount of research has been conducted
since the early 1990s, including case studies that analyze the transfor-
mation or stability of electoral systems (Mainwaring 1999; Siavelis 1997),
as well as data accumulation regarding the different dimensions of elec-
toral rules, including electoral formulas, electoral thresholds, district
magnitudes, and ballot structures (Colomer 2004; Coppedge 1997;
Golder 2003; Jones 1995a, b; Lundell and Karvonen 2003; Nohlen 1993,
2005b; Wallack et al. 2003).
34 LATIN AMERICAN POLITICS AND SOCIETY 51: 3