Predictive checks of the three effective spin models explored in Section 4. Under each model, we repeatedly generate sets of “Observed” χ _eff values drawn from our binary black hole observations, and sets of “Predicted” values according to the fitted models; see Appendix C for further details. Each trace among the three subplots represents one such realization, with the spread among traces reflecting both our uncertainty in the properties of each observed binary as well as the uncertainty in model hyperparameters. A model that provides a good fit to observation will yield an ensemble of traces centered symmetrically around the diagonal, whereas any systematic departure from the diagonal would indicate model failure. No such departures are seen, and so all three effective spin models are able to successfully reproduce observation.