  The LIGO-Virgo collaboration recently reported the properties of GW190412, a
binary black hole merger with unequal component masses (mass ratio
$0.25^{+0.06}_{-0.04}$ when using the EOBNR PHM approximant) and a
non-vanishing effective spin aligned with the orbital angular momentum. They
used uninformative priors to infer that the more massive black hole had a
dimensionless spin magnitude between 0.17 and 0.59 at 90\% confidence. We argue
that, within the context of isolated binary evolution, it is more natural to
assume a priori that the first-born, more massive black hole has a negligible
spin, while the spin of the less massive black hole is preferentially aligned
with the orbital angular momentum if it is spun up by tides. Under this
astrophysically motivated prior, we conclude that the lower mass black hole had
a dimensionless spin component between 0.64 and 0.99 along the orbital angular
momentum.
