Dissertation Title: Language Models as Opinion Models: Techniques and Applications

Abstract: 

Real-time social media platforms now host the news cycle and shape public opinion, while large language models (LLMs) give us new tools to observe and predict those shifts. This dissertation links the new affordances of social media with the predictive power of LLMs to explain—and forecast—opinion change. We first quantify the dynamics of news on an influential social platform, then develop LLM-based tools to forecast persuasion and predict heterogeneous treatment effects (HTEs).

Study I — Media tempo and tone. Using 518k hours of U.S. talk-radio broadcasts and 26.6 million tweets from elite and mass users, we show that Twitter discourse (i) moves faster at both take-off and fade-out stages of a news event and (ii) sustains greater outrage than radio – despite radio’s often explicitly outrage-focused business model. To our knowledge, this is the first large-scale, data-driven comparison of outrage levels between Twitter and traditional media.

Study II — Zero-shot persuasion forecasting. Across a diverse set of 28 randomized experiments, LLM-based methods outperform an ensemble of strong baselines at predicting HTEs and deliver good performance at predicting average treatment effects (ATEs) — all without any experiment-specific fine-tuning.

Study III — Transfer and scaling. Fine-tuning LLMs on contemporaneous news coverage boosts HTE (and ATE) prediction performance greatly, to more than 3x baseline performance. A new minibatch-moment-matching (M3) objective lets us train a 400M-parameter model to nearly match the HTE pre-diction performance of an 8B model at a fraction of the inference cost. Transfer, however, falters out of distribution on held-out experiments and demographic groups, lending support to contextual theories of persuasion.

Overall, we (i) quantify how platform affordances shape the tone and tempo of public discourse, (ii) introduce LLM-based methods that make causal experiments more sample-efficient, and (iii) chart the limits of transfer learning for opinion prediction. Our findings provide practical tools for HTE prediction and help researchers anticipate persuasion dynamics in a media landscape shaped by both humans and machines.

Deb Roy, Professor of Media Arts and Sciences, MIT (chair)
Jacob Andreas, Associate Professor of EECS, MIT
John Horton, Associate Professor of Information Technology, MIT Sloan School of Business