TERRA: A Transformer-Enabled Recursive R-learner for Longitudinal Heterogeneous Treatment Effect Estimation

Accurately estimating heterogeneous treatment effects (HTE) in longitudinal settings is essential for personalized decision-making across healthcare, public policy, education, and digital marketing. However, time-varying interventions introduce many unique challenges, such as carryover effects, time-varying heterogeneity, and post-treatment bias, which are not addressed by standard HTE methods. To address these challenges, we introduce TERRA (Transformer-Enabled Recursive R-learner), which facilitates longitudinal HTE estimation with flexible temporal modeling and learning. TERRA has two components. First, we use a Transformer architecture to encode full treatment-feature histories, enabling the representation of long-range temporal dependencies and carryover effects, hence capturing individual- and time-specific treatment effect variation more comprehensively. Second, we develop a recursive residual-learning formulation that generalizes the classical structural nested mean models (SNMMs) beyond parametric specifications, addressing post-treatment bias while reducing reliance on functional assumptions. In simulations and data applications, TERRA consistently outperforms strong baselines in HTE estimation in both accuracy and stability, highlighting the value of combining principled causal structure with high-capacity sequence models for longitudinal HTE.