Inferred evolution of the merger-rate density ﹩{ \mathcal R }({\chi }_{{\rm{eff}}};z)﹩ over redshift and as a function of the effective spin χ_eff ≈ 0 (top), 0.05 (middle), and 0.2 (bottom). The inferred median and central 90% credible region are given by the solid line and shaded areas, respectively, while the underlying evolution of the true population is given by the dashed black line.