Large language models that replace human participants can harmfully misportray and flatten identity groups

Large language models (LLMs) are increasing in capability and popularity, propelling their application in new domains—including as replacements for human participants in computational social science, user testing, annotation tasks and so on. In many settings, researchers seek to distribute their surveys to a sample of participants that are representative of the underlying human population of interest. This means that to be a suitable replacement, LLMs will need to be able to capture the influence of positionality (that is, the relevance of social identities like gender and race). However, we show that there are two inherent limitations in the way current LLMs are trained that prevent this. We argue analytically for why LLMs are likely to both misportray and flatten the representations of demographic groups, and then empirically show this on four LLMs through a series of human studies with 3,200 participants across 16 demographic identities. We also discuss a third limitation about how identity prompts can essentialize identities. Throughout, we connect each limitation to a pernicious history of epistemic injustice against the value of lived experiences that explains why replacement is harmful for marginalized demographic groups. Overall, we urge caution in use cases in which LLMs are intended to replace human participants whose identities are relevant to the task at hand. At the same time, in cases where the benefits of LLM replacement are determined to outweigh the harms (for example, engaging human participants may cause them harm, or the goal is to supplement rather than fully replace), we empirically demonstrate that our inference-time techniques reduce—but do not remove—these harms. Large language models are being considered to simulate responses from participants of different backgrounds in computational social science experiments. Here it is shown that this practice can misportray and flatten demographic groups in distinctively harmful ways.