  The astrophysical origins of the binary black hole systems seen with
gravitational waves are still not well understood. However, features in the
distribution of black-hole masses, spins, redshifts, and eccentricities provide
clues into how these systems form. Much has been learned by investigating these
distributions one parameter at a time. However, we can extract additional
information by studying the covariance between pairs of parameters. Previous
work has shown preliminary support for an anti-correlation between mass ratio
$q \equiv m_2/m_1$ and effective inspiral spin $\chi_\mathrm{eff}$ in the
binary black hole population. In this study, we test for the existence of this
anti-correlation using updated data from the third gravitational wave transient
catalogue (GWTC-3) and improve our copula-based framework to employ a more
robust model for black-hole spins. We find evidence for an anti-correlation in
$(q, \chi_\mathrm{eff})$ with 99.7% credibility. This may imply high
common-envelope efficiencies, stages of super-Eddington accretion, or a
tendency for binary black hole systems to undergo mass-ratio reversal during
isolated evolution. Covariance in $(q,\chi_\mathrm{eff})$ may also be used to
investigate the physics of tidal spin-up as well as the properties of
binary-black-hole-forming active galactic nuclei.
