キャス・サンスティーン（Cass R. Sunstein、1954年–）は、憲法、行政法、法哲学と行動経済学を研究する法学者。1975年にハーバード大学を卒業し、1978年に同大学ロースクールを修了した。シカゴ大学での研究・教育を経てハーバード大学に移り、法制度と人間の判断、規制の設計、民主政治における情報や集団の影響を研究した。2009年から2012年には、オバマ政権のホワイトハウス情報・規制問題室長として連邦規制の審査に携わった。2018年には、法学と社会科学にまたがる研究によりホルベア賞を受賞した。リチャード・セイラーとの共著『実践 行動経済学』（原題 “Nudge: Improving Decisions About Health, Wealth, and Happiness”）では、人が合理的な判断から外れることを出発点に、選択の自由を保ちながら意思決定を支える「選択アーキテクチャー」を論じた。2021年の完全版では、技術や制度の変化を踏まえて内容を更新し、日本語では2022年に刊行された。『スラッジ――不合理をもたらすぬかるみ』（原題 “Sludge: What Stops Us from Getting Things Done and What to Do about It”）では、複雑な申請書類や解約しにくいサービスなど、必要な行動を妨げる摩擦に焦点を移し、その負担と削減策を検討した。2026年の『Separation of Powers: How to Preserve Liberty in Troubled Times』（未邦訳）では、自由と自治を支える権力分立を取り上げた。ホワイトハウスや司法省などでの経験を踏まえ、立法・行政・司法の権限を分ける原理と、権力の集中を防ぐ仕組みを説明した。
Cass R. Sunstein (born 1954) is a legal scholar whose research spans constitutional and administrative law, legal philosophy, and behavioral economics. He graduated from Harvard College in 1975 and Harvard Law School in 1978. After teaching and conducting research at the University of Chicago, he joined Harvard University. His work examines relationships between legal institutions and human judgment, the design of regulation, and the effects of information and groups on democratic politics. From 2009 to 2012, he served as Administrator of the White House Office of Information and Regulatory Affairs in the Obama administration, reviewing federal regulations. In 2018, he received the Holberg Prize for research bridging law and the social sciences. With Richard H. Thaler, he wrote “Nudge: Improving Decisions About Health, Wealth, and Happiness,” which began with the observation that people often depart from rational decision-making. It developed the idea of choice architecture: designing the circumstances of decisions to support better choices while preserving freedom to choose. The Final Edition, published in 2021 and translated into Japanese in 2022, updated the discussion in light of technological and institutional change. “Sludge: What Stops Us from Getting Things Done and What to Do about It” shifted attention to frictions that obstruct action, including burdensome application forms and services that are difficult to cancel. It examined the costs of these obstacles and ways to reduce them. In “Separation of Powers: How to Preserve Liberty in Troubled Times” (2026), Sunstein investigated the division of government power as a foundation of liberty and self-government. Drawing on experience in institutions including the White House and the Department of Justice, he explained principles separating legislative, executive, and judicial authority and mechanisms for resisting the concentration of power.
