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Properties of the to-date-observed binary black hole (BBH) merger eventssuggest a preference towards spin-orbit aligned mergers. Naturally, this hascaused widespread interest and speculations regrading implications on variousmerger formation channels. Here we show that (i) not only the BBH-mergerpopulation from isolated binaries, but also (ii) BBH population formed in youngmassive clusters (YMC) would possess an asymmetry in favour of aligned mergers,in the distribution of the events' effective spin parameter ($\chi_{\rm eff}$).In our analysis, we utilize BBH-merger outcomes from state-of-the-art N-bodyevolutionary models of YMCs and isolated binary population synthesis. Weincorporate, for the first time in such an analysis, misalignments due to bothnatal kicks and dynamical encounters. The YMC $\chi_{\rm eff}$ distribution hasa mean (an anti-aligned merger fraction) of $\langle\chi_{\rmeff}\rangle\leq0.04$ ($f_X-\approx40\%$), which is smaller (larger) than butconsistent with the observed asymmetry of $\langle\chi_{\rmeff}\rangle\approx0.06$ ($f_X-\approx28\%$) as obtained from the populationanalysis by the LIGO-Virgo-KAGRA collaboration. In contrast, isolated binariesalone tend to produce a much stronger asymmetry; for the tested physicalmodels, $\langle\chi_{\rm eff}\rangle\approx0.25$ and $f_X-\lesssim7\%$.Although the YMC $\chi_{\rm eff}$ distribution is more similar to the observedcounterpart, none of the channels correctly reproduce the observeddistribution. Our results suggest that further extensive model explorations forboth isolated-binary and dynamical channels as well as better observationalconstraints are necessary to understand the physics of 'the symmetry breaking'of the BBH-merger population.

Symmetry breaking in merging binary black holes from young massive clusters and isolated binaries