This Article introduces the generative reasonable person, a new tool for estimating how ordinary people judge reasonableness. As claims about AI capabilities often outpace evidence, the Article proceeds empirically: adapting randomized controlled trials to large language models, it replicates three published studies of lay judgment across negligence, consent, and contract interpretation, drawing on nearly 10,000 simulated decisions. The findings reveal that models can replicate subtle patterns that run counter to textbook treatment. Like human subjects, models prioritize social conformity over cost-benefit analysis when assessing negligence, inverting the hierarchy that textbooks teach. They reproduce the paradox that material lies erode consent less than lies about a transaction's essence. And they track lay contract formalism, judging hidden fees more enforceable than fair. For two centuries, scholars have debated whether the reasonable person is empirical or normative, majoritarian or aspirational. But much of this debate assumed a constraint that no longer holds: that lay judgments are expensive to surface, slow to collect, and unavailable at scale. Generative reasonable people loosen that constraint. They offer judges empirical checks on elite intuition, give resource-constrained litigants access to simulated jury feedback, and let regulators pilot-test public comprehension, all at a fraction of survey costs. The reasonable person standard has long functioned as a vessel for judicial intuition precisely because the empirical baseline was missing. With that baseline now available, departures from lay understanding become transparent rather than hidden, a choice to be justified, not a fact to be assumed. Properly cabined, the generative reasonable person may become a dictionary for reasonableness judgments.

翻译：本文提出“生成式理性人”这一新工具，用于评估普通民众如何判断合理性。鉴于关于人工智能能力的宣称常超越现有证据，本文采用实证研究方法：通过将随机对照试验适配于大语言模型，复现了已发表的关于过失、同意与合同解释领域的三项公众判断研究，共基于近万次模拟决策。研究发现，模型能够复现与教科书处理方式相悖的微妙模式。与人类受试者类似，模型在评估过失时优先考虑社会一致性而非成本效益分析，从而颠覆了教科书所教授的层级关系。模型再现了实质性谎言比交易本质谎言更少削弱同意效力的悖论。同时模型追踪公众的合同形式主义倾向，认定隐性费用比公平条款更具执行力。两个世纪以来，学者们持续争论理性人标准应属实证性或规范性、多数主义或理想主义。但此类争论大多基于一个已不成立的前提：公众判断的呈现成本高昂、收集过程缓慢且难以规模化获取。生成式理性人松绑了这一限制。它们为法官提供检验精英直觉的实证基准，使资源有限的诉讼当事人获得模拟陪审团反馈，并允许监管机构以远低于调研成本的方式预测试公众理解。理性人标准长期作为司法直觉的载体，正是因为缺乏实证基线。随着该基线的建立，偏离公众认知的行为将从隐性转为透明，成为需要论证的选择而非默认的事实。在合理约束下，生成式理性人或将成为合理性判断的词典。