The Hierarchical Merger Scenario for GW231123

GW231123 exhibits exceedingly massive components and high spins, which challenges the formation of first-generation (1G) black holes from stellar collapse and implies that this event might originate from hierarchical mergers. Here we show that the $\mathtt{\text{2G+2G}}$ merger scenario for GW231123 is favored over a $\mathtt{\text{2G+1G}}$ (or $\mathtt{\text{3G+2G}}$) merger, with odds ratios of $>$$\mathcal{O}({10}^3)$. The primary (secondary) black hole is consistent with merging binary black holes with masses of $75_{-7}^{+7}{M}_{\odot }$ and $66_{-10}^{+4}{M}_{\odot }$ ($64_{-10}^{+8}{M}_{\odot }$ and $59_{-9}^{+7}{M}_{\odot }$; 90% credible intervals), respectively. Our results reveal that the treatment of spin priors from the population level and waveform model choice are decisive in interpreting potential hierarchical gravitational-wave signals.