The Trouble with Rational Expectations in Heterogeneous Agent Models: A Challenge for Macroeconomics

• 成果类型: Article
• 署名作者: Moll, Benjamin
• 署名单位: University of London; London School Economics & Political Science
• 刊物名称: ECONOMIC JOURNAL
• ISSN/ISSBN: 0013-0133
• DOI: 10.1093/ej/ueaf104
• 发表日期: 2026
• 关键词: REWARD PREDICTION ERROR; natural expectations; idiosyncratic risk; monetary-policy; equilibrium; uncertainty; INFORMATION; prices; WEALTH; income

摘要

The thesis of this essay is that, in heterogeneous agent macroeconomics, the assumption of rational expectations about equilibrium prices is unrealistic and should be replaced. Rational expectations imply that decision-makers forecast equilibrium prices like interest rates by forecasting cross-sectional distributions. This leads to an extreme version of the curse of dimensionality: dynamic programming problems in which the entire distribution is a state variable (the 'Master equation', also known as the 'Monster equation'). Frontier computational methods struggle with these infinite-dimensional Bellman equations, making it implausible that real-world agents solve the associated decision problems. These difficulties also limit the applicability of the heterogeneous agent approach to central questions in macroeconomics-those involving aggregate risk and non-linearities such as financial crises. This troublesome feature of the rational expectations assumption poses a challenge: what should replace it? I outline three criteria for alternative approaches: (1) computational tractability, (2) consistency with empirical evidence and (3) (some) immunity to the Lucas critique. I then discuss several promising directions, including temporary equilibrium approaches, incorporating survey expectations, least-squares learning and reinforcement learning.