High-χ_eff fractions in the underlying distribution inferred from the population fit described in Sect. 3.3, in blue and orange according to the legend. Lighter contour colors indicate larger CIs of 50% and 90%, respectively. The fraction of BBH systems with large positive spins in the underlying population may increase with increasing redshift (credibility 82%), consistent with our model predictions (black). We do not yet have enough BBH events at z ∼ 1 to accurately measure the χ_eff distribution at high z and therefore cannot confidently conclude that the distribution is evolving.