  The Advanced LIGO and Virgo detectors have now observed approximately 50
black-hole-binary mergers, from which we can begin to infer how rapidly
astrophysical black holes spin. The LIGO-Virgo Collaboration (LVC) analysis of
detections up to the end of the first half of the third observing run (O3a)
appeared to uncover a distribution of spin magnitudes that peaks at $\sim$0.2.
This is surprising: is there a black-hole formation mechanism that prefers a
particular, non-zero spin magnitude, or could this be the cumulative effect of
multiple formation processes? We perform an independent analysis of the most
recent gravitational-wave catalogue, and find that (a) the support for the LVC
spin-magnitude is tenuous; in particular, adding or removing just one signal
from the catalogue can remove the statistical preference for this distribution,
and (b) we find potential evidence for two spin sub-populations in the observed
black holes; one with extremely low spins and one with larger spin magnitudes.
We make the connection that these spin sub-populations could be correlated with
the mass of the binary, with more massive binaries preferring larger spin
magnitudes, and argue that this may provide evidence for hierarchical mergers
in the second gravitational-wave catalogue.
