  Recent work paints a conflicting portrait of the distribution of black hole
spins in merging binaries measured with gravitational waves. Some analyses find
that a significant fraction of merging binaries contain at least one black hole
with a spin tilt $>90^\circ$ with respect to the orbital angular momentum
vector, which has been interpreted as a signature for dynamical assembly. Other
analyses find the data are consistent with a bimodal population in which some
binaries contain black holes with negligible spin while the rest contain black
holes with spin vectors preferentially aligned with the orbital angular
momentum vector. In this work, we scrutinize models for the distribution of
black hole spins to pinpoint possible failure modes in which the model yields a
faulty conclusion. We reanalyze data from the second LIGO--Virgo
gravitational-wave transient catalog (GWTC-2) using a revised spin model, which
allows for a sub-population of black holes with negligible spins. In agreement
with recent results by Roulet et al., we show that the GWTC-2 detections are
consistent with two distinct sub-populations. We estimate that $29-75\%$ (90\%
credible interval) of merging binaries contain black holes with negligible spin
$\chi \approx 0$. The remaining binaries are part of a second sub-population in
which the spin vectors are preferentially (but not exactly) aligned to the
orbital angular momentum. The black holes in this second sub-population are
characterized by spins of $\chi\sim0.45$. We suggest that the inferred spin
distribution is consistent with the hypothesis that all merging binaries form
via the field formation scenario.
