Cumulative probability distributions of massive binary black hole merger properties for galaxies simulated with CDM (purple) and SIDM-1 (green). Left: cumulative massive binary merger probability as a function of cosmic time. Note that the CDM model exhibits more rapid assembly of massive black holes by mergers at early times. We show the medians with dotted lines, and the third-quartiles with dashed lines. Right: cumulative massive binary merger probability as a function of mass ratio. The SIDM-1 model favors moderate mass ratio mergers, while nearly half the mergers in the CDM model are nearly equal mass.