We investigate the systematic biases in measuring orbital eccentricity for binary black hole (BBH) mergers that arise when higher-order modes (HOMs) of gravitational waves are neglected in waveform modeling. Using Bayesian inference with the state-of-the-art eccentric, spin-aligned, higher-mode effective-one-body model seobnrv5ehm, we reanalyze six previously suggested eccentric gravitational-wave events—GW190521, GW190620, GW190701, GW191109, GW200129, and GW200208_222617. Comparing results with its dominant-mode-only counterpart seobnrv5e, we find no statistically significant HOM-induced bias in eccentricity for any of these events, including GW190521, whose eccentricity has been debated in the literature. To explore representative parameter regimes vulnerable to HOM omission, we perform a broad zero-noise injection campaign varying detector-frame total mass, mass ratio, eccentricity, inclination, and network SNR. We find that significant systematic biases (${\mathrm{\Delta }}_e/\sigma >1$) arise predominantly in systems with relatively high total mass ($M^{\det }\gtrsim 120M_{\odot }$), highly asymmetric mass ratios ($q\gtrsim 4$), large inclinations (${\theta }_{\mathrm{JN}}\gtrsim 30°$), and high SNRs (${\rho }_{\mathrm{mf}}^N\approx 50$). Notably, for quasicircular BBHs with approximately $M^{\det }\gtrsim 140M_{\odot }$, our results suggest that neglecting HOMs may lead to strong false-positive evidence for nonzero eccentricity. By contrast, for lower-mass systems ($M^{\det }\sim 100M_{\odot }$), HOM exclusion produces negligible eccentricity biases. Our results demonstrate that although current eccentric candidates are not impacted by HOM omission, future eccentricity measurements—particularly for relatively massive, asymmetric, or edge-on systems—require HOM-inclusive waveforms to avoid substantial systematic errors.

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• Gravitational wave detection
• Gravitational waves
• Astronomical black holes
• Binary stars
• Numerical simulations in gravitation & astrophysics