In Agents We Trust, but Who Do Agents Trust? Latent Source Preferences Steer LLM Generations

Large Language Model (LLM) based agents are increasingly being deployed as user-friendly front-ends on online platforms, where they filter, prioritize, and recommend information retrieved from the platforms' back-end databases or via web search. In these scenarios, LLM agents act as decision assistants, drawing users' attention to particular instances of retrieved information at the expense of others. While much prior work has focused on biases in the information LLMs themselves generate, less attention has been paid to the factors and mechanisms that determine how LLMs select and present information to users. We hypothesize that when information is attributed to specific sources (e.g., particular publishers, journals, or platforms), LLMs will exhibit systematic latent source preferences. That is, they will prioritize information from some sources over others based on attributes such as the sources' brand identity, reputation, or perceived expertise, encoded within their parametric knowledge. Through controlled experiments on twelve LLMs from six model providers, spanning both synthetic and real-world tasks including news recommendation, research paper selection, and choosing e-commerce platforms, we find that several models consistently exhibit strong and predictable source preferences. These preferences are sensitive to contextual framing, can outweigh the influence of content itself, and persist despite explicit prompting to avoid them. They also help explain phenomena such as the observed left-leaning skew in news recommendations, which arises from higher trust in certain sources rather than the content itself. Our findings advocate for deeper investigation into the origins of these preferences during pretraining, fine-tuning and instruction tuning, as well as for mechanisms that provide users with transparency and control over the biases guiding LLM-powered agents.