  GW190412 is the first observation of a black hole binary with definitively
unequal masses. GW190412's mass asymmetry, along with the measured positive
effective inspiral spin, allowed for inference of a component black hole spin:
the primary black hole in the system was found to have a dimensionless spin
magnitude between 0.17 and 0.59 (90% credible range). We investigate how the
choice of priors for the spin magnitudes and tilts of the component black holes
affect the robustness of parameter estimates for GW190412, and report Bayes
factors across a suite of prior assumptions. Depending on the waveform family
used to describe the signal, we find either marginal to moderate (2:1-6:1) or
strong ($\gtrsim$ 20:1) support for the primary black hole being spinning
compared to cases where only the secondary is allowed to have spin. We show how
these choices influence parameter estimates, and find the asymmetric masses and
positive effective inspiral spin of GW190412 to be qualitatively, but not
quantitatively, robust to prior assumptions. Our results highlight the
importance of both considering astrophysically motivated or population-based
priors in interpreting observations and considering their relative support from
the data.
