Transitions in the Mass-ratio and Spin Properties of Binary Black Holes in GWTC-5

We analyze the mass-ratio and effective-spin ( χ eff ) distributions of binary black hole mergers in the latest gravitational-wave catalog, GWTC-5, as a function of primary mass. Using hierarchical Bayesian inference with flexible Gaussian-process population models, we identify four distinct mass regions separated by transitions in both mass-ratio and spin properties. Below ∼18 M ⊙ the population strongly favors equal-mass binaries and exhibits a narrow, symmetric χ eff distribution peaked at positive values. The positive skewness previously reported for the overall population is absent from this low-mass component, and it is most likely associated with higher-mass binaries. In the range 18–30 M ⊙ , the mass-ratio distribution becomes substantially flatter, while the χ eff distribution broadens and shifts to a peak consistent with zero. The region associated with the feature near ≃35 M ⊙ returns to a narrow χ eff distribution consistent with symmetry at zero and strongly favors equal-mass binaries. Above ≃45 M ⊙ , both the mass-ratio and χ eff distributions broaden significantly. The inferred support of the spin distribution converges toward the range expected for binaries containing remnants of previous black hole mergers, making the highest-mass region fully consistent with a star cluster population of hierarchical mergers. The close correspondence between transitions in mass ratio and effective spin suggests that different primary-mass ranges trace distinct formation channels, with isolated binary or triple evolution likely dominating the lower-mass population and dynamical assembly becoming increasingly important at higher masses.