Building better spin models for merging binary black holes

The spins of black holes in merging binaries are a key probe of astrophysical processes ranging from supernova mechanisms to the dynamics of dense environments. However, individual measurements of black hole spins from gravitational-wave observations have large uncertainties. We show that previous inferences about the astrophysical distribution of black hole spins are strongly dependent on modeling assumptions. Specifically, we should that contrary to previous reported results, all observed black holes are consistent with either having negligible spins or spins aligned with the orbital angular momentum.