Fig. 2
From: Congressional symmetry: years remaining mirror years served in the U.S. House and Senate
Stationary Population Identity (SPI) model (see Fig. 1) conceived as a Congressional Life Cycle (CLC) model. The green-shaded area indicates the distribution of years served (left) and years remaining (right) by the population at the midpoint with the terms of the predecessors they replaced immediately to the left and the successors who replaced them immediately to the right. Formation begins retrospectively at the election of the most senior member, time TF 50 years earlier, membership grows incrementally for the next 50 years through completion at time TC and then senesces for the following 50 years to extinction at time TE when the last member leaves office. Note that the birth rate of the incumbents (bI) equal the death rates of their predecessors (dP) in the Growth Phase, and that the death rate of the incumbents (dI) equals the birth rate of their successors (bS) in the Senescence Phase. Also note that each “step” (new election) in the Growth and Senescence phases represents the transition of a Senate seat FROM one party TO either the same party or the opposing one. There are thus 100 party transitions in each phase spanning 40 or more years and 200 total party transitions across the entire CLC spanning 80 or more years