Recent large-scale T2I models like DALLE-3 have made progress on improving fairness in single-subject generation, i.e. generating a one-person image. However, we reveal that these improved models still demonstrate considerable biases when simply generating two people. To systematically evaluate T2I models in this challenging generation setting, we propose the Paired Stereotype Test (PST) framework, established as a dual-subject generation task, i.e. generating two people in the same image. The setting in PST is especially challenging, as the two individuals are described with social identities that are male-stereotyped and female-stereotyped, respectively, e.g. "a CEO" and "an Assistant". It is easy for T2I models to unfairly follow gender stereotypes in this contrastive setting. We establish a metric, Stereotype Score (SS), to quantitatively measure the adherence to gender stereotypes in generated images. Using PST, we evaluate two aspects of gender biases in DALLE-3 -- the widely-identified bias in gendered occupation, as well as a novel aspect: bias in organizational power. Results show that despite generating seemingly fair or even anti-stereotype single-person images, DALLE-3 still shows notable biases under PST -- for instance, in experiments on gender-occupational stereotypes, over 74% model generations demonstrate biases. Moreover, compared to single-person settings, DALLE-3 is more likely to perpetuate male-associated stereotypes under PST. Our work pioneers the research on bias in dual-subject generation, and our proposed PST framework can be easily extended for further experiments, establishing a valuable contribution.

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