More general reviews are also analyzed of third-party history by Steven Rosenstone, Roy Behr, and Edward Lazarus (1984), John Bibby and Sandy Maisel (1998) and David Gillepsie (1993). Lastly, dissertations by Joan Bryce (1996) and Jimmie McClellan (1984) that categorize the barriers for American third parties are combined with the literature on individual roadblocks and an overview by Theodore Lowi (1998) to present an overall picture of the barriers facing third parties in the United States.
Sociological and an institutional framework are the two explanations for the barriers that exist for political parties in the United States. The focus of this paper is on the institutional barriers that are confronted by third party candidates in presidential politics. I am not saying that social factors should not be contemplated. An emphasis on the interaction of social and institutional factors seems to be the most plausible approach. .
An interactive model assuming the need for both heterogeneity and proportional electoral laws is more predictive than an additive model. According to Neto and Cox:.
"A polity can tend towards bipartism either because it has a strong electoral system or because it has few cleavages. Mutipartism arises as the joint product of many exploitable cleavages and a permissive electoral system."" .
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A predictive model using only institutional variables explains sixty one percent (61%) of the variation in the effective number of parties among democracies as Neto and Cox have proven in their research. The model is substantially improved by combining the variables with ethnic heterogeneity. Cox continues:.
"Social cleavages thus seem to play no systematic role in determining the equilibrium number of parties. They do play a residual role."" .
Cox concludes:.
"Increasing the diversity of the social structure in a non-proportional electoral system does not pro-liferate parties, whereas it does in a proportional system.