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This paper explores the causes of disproportionality — the difference between the percentage of seats and percentage of votes obtained by a party — in 18 Latin-American countries, by measuring the effect of four independent variables. The first part presents the research design, the main theoretical approaches used to study electoral systems and a literature review, where both disproportionality and the independent variables are conceptualized. The second part discusses the general behavior of electoral systems and the independent variables in Latin America. The third part contains the inferential analysis, where we measure the impact of the independent variables on disproportionality. We find that there is a direct relationship between district magnitude and disproportionality. Moreover, we find that the higher the threshold of exclusion — the number of voters that could potentially be left without representation because of the district magnitude and the electoral formula —, the higher the disproportionality. We find that proportional electoral systems, rather than majority or mixed electoral systems, tend to produce less distortion between the percentage of votes and the percentage of seats. We find that a low fragmentation in the party system — that is, a small number of parties with a presence in Congress — weakly contributes to increase disproportionality. Finally, we find that malapportionment (i.e., the disproportionate distribution of seats based on the number of voters in that district) has no statistically significant relationship with disproportionality. [ABSTRACT FROM AUTHOR]