Marginal posterior distribution for χ _eff using results from Abbott et al. (2021b) that have no excess of zero-spin events (blue) and results from Galaudage et al. (2021) that imply that ∼50% of binaries have zero spins (red). The solid lines trace the mean distribution inferred by each model while shaded regions denote the 90% credible regions; the light traces trace individual draws from the population posterior under each model. We see that, if a significant fraction of binary black holes are indeed nonspinning, this should be clearly identifiable through their χ _eff distribution. This figure is modeled after Figure 5 of Galaudage et al. (2021).