Seat share distribution of parties: Models and empirical patterns

We find that strategic sequencing and other factors sort parties roughly into two groups. Low-ranking parties lose part of their inherent support, compared to probabilistic expectations, while high-ranking parties profit from the shift. Our method is to graph the worldwide mean seat shares of parties at various ranks by size against the largest party share (Nagayama triangle format). The resulting empirical pattern looks complex, yet when we adjust a probabilistic model to account for strategic and other factors that may hurt the smaller parties, the fit becomes close. The number of parties that profit from transfers is close to the inverse of the fractional share of the largest party. The model fits best when the transfer is assumed to involve about one-half of inherent minor party support. This is a novel way to estimate the universal average strength of strategic and other factors that work against the smaller parties. The empirical worldwide mean pattern offers us a norm against which seat share distributions in individual countries or single elections can be compared.