A dynamic seat product model
2026-07-31
The Seat Product Model uses just two institutional parameters (average district magnitude and assembly size) to predict legislative fragmentation with remarkable accuracy. However, it is best suited for simple, single-tier systems. Subsequent extensions to complex, two-tier systems rely on an upper-tier (multiplier-style) correction that accounts for cross-tier inflationary effects but estimate it as a single constant from pooled cross-national data. Applied to Mexico’s mixed-member lower house elections (1997–2021), both specifications systematically mispredict legislative fragmentation. This is because cross-tier inflationary effects are not a fixed system property but an evolving one, shaped by institutional features interacting with a changing party system. This study proposes a Dynamic Seat Product Model that retains the original institutional baseline while allowing the correction to update automatically via exponential smoothing. The DSPM tracks Mexico’s long-run rise in legislative fragmentation more closely than fixed alternatives, clarifies the mechanism behind it, and offers a principled path toward making the SPM applicable to virtually any single election.