Legislature Size and Party Unity: Evidence from Historical US State Legislatures

The size of a legislature is one of its most fundamental features, yet there is little consensus on how large legislatures should be. Furthermore, many modern-day reformers are dissatisfied with the current size of American legislatures. We explore the effect of legislature size on party unity, which is important for policy outcomes and is plausibly shaped through both individual and collective incentives affected by legislature size. To test this relationship, we use original roll-call data from the Illinois House of Representatives in the mid-nineteenth century, which dramatically changed size twice in this period. Using Indiana as a control, we show that larger chambers are associated with lower party unity and that this change is likely due to intralegislature dynamics rather than legislator–constituent relationships. We demonstrate that our results extend to more-modern periods by using additional data from Vermont and New Jersey.