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

Black-hole mergers in disklike environments could explain the observed $q$-$\chi_\mathrm{eff}$ correlation