  Current gravitational-wave data from stellar-mass black-hole binary mergers
suggest a correlation between the binary mass ratio $q$ and the effective spin
$\chi_\mathrm{eff}$: more unequal-mass binaries consistently show larger and
positive values of the effective spin. Multiple generations of black-hole
mergers in dense astrophysical environments may provide a way to form
unequal-mass systems, but they cannot explain the observed correlation on their
own. We show that the symmetry of the astrophysical environment is a crucial
feature to shed light on this otherwise puzzling piece of observational
evidence. We present a toy model that reproduces, at least qualitatively, the
observed correlation. The model relies on axisymmetric, disk-like environments
where binaries participating in hierarchical mergers share a preferential
direction. Migration traps in AGN disks are a prime candidate for this setup,
hinting at the exciting possibility of constraining their occurrence with
gravitational-wave data.
