Abstract: Using the General Leftism scale developed in Bronski 2023 [1], we ask a 333 person sample on Prolific to fill out the scale both for themselves and their spouse. Using a narrow-sense heritability estimate of 0.6, we find a selection pressure of 0.076 SDs per generation in the conservative direction. Then, we examine data from Bronski 2023 [1] that allows us to compute the mutational pressure as 0.275 SDs in the leftist direction. We find that the sum of these two pressures adequately explains the change in general leftism per generation over that last 70 years (0.18 SD per generation in the leftist direction), indicating that Western political change is solely due to evolutionary pressure. Per Bayesian analysis, there is a 95% chance, given this data, that 70% or more of the observed shift in leftism is due solely to evolutionary pressure, namely mutational pressure.