Human raters called more attractive faces higher quality even against real photos, and the same bias showed up inside four of five reward models
AspireAI community · arXiv (Mannering, Huang, Prügel-Bennett)
Text-to-image evaluation now runs through human preference, and this work shows human preference carries a bias that leaks into the leaderboards built on it. In a controlled user study, annotators ranked more attractive faces as higher quality, even when an unattractive real photograph was compared against an attractive AI-generated face. Simulating Elo rankings from that study data, attractiveness could distort a leaderboard's ordering enough to lift synthetic attractive faces above real unattractive ones. The bias also reappears inside the reward models: editing 40 annotated facial attributes in isolation, attractiveness was the strongest correlate of score for four of five human preference models (PickScore, MPS, HPSv2.1 and HPSv3) and the strongest predictor in regression. Preference-aligned generation also shifts faces toward younger and more feminine ones and can override an explicit prompt asking for unattractive faces. The limitation is scope - it is about human faces, and the paper says the mitigation strategies for data collection and post-training are explored rather than settled.
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