Presidential coattails and legislative fragmentation

Considerable evidence suggests that legislative fragmentation can negatively affect the survival of democratic presidential regimes. While there is a vast literature examining the determinants of legislative fragmentation, one factor that has traditionally been overlooked is the impact of presidential elections. Do presidential elections increase or decrease legislative fragmentation? Does it matter if presidents are elected by plurality rule or by runoff? Using a new dataset that covers all democratic legislative and presidential elections between 1946 and 2000, I find that presidential coattails can reduce, increase, or have no effect on legislative fragmentation depending on the number of presidential candidates. I also find strong evidence that social heterogeneity increases the number of presidential candidates when runoff systems are employed. Taken together, these results suggest that the widespread adoption of runoffs by newly democratic presidential regimes will likely increase legislative fragmentation, thereby putting their democratic survival at increased risk.