Playing with Trolleys: Intuitions About the Permissibility of Aggregation


  • Mark Kelman,

    Corresponding author
    • Address correspondence to Mark Kelman, James C. Gaither Professor of Law and Vice Dean, Stanford Law School, Crown Quadrangle, 559 Nathan Abbott Way, Stanford, CA 94305; email:

    Search for more papers by this author
  • Tamar Admati Kreps


We explore when experimental subjects think aggregation across persons—deciding that some parties should be worse off, so that others might gain more, compared to an alternative option—is permissible through subjects' responses to trolley problem vignettes. Two classic vignettes are: (1) whether to divert a runaway trolley on to a spur track, killing one, to save multiple potential victims on a main track, and (2) whether to push an overweight person off a bridge to block the trolley from hitting the potential victims. Subjects typically judge diverting in the first problem permissible and pushing in the second problem impermissible. We first show that respondents reject Taurek's claim that the number of victims is irrelevant. Our central novel finding is that the intuition to distinguish the two cases is weaker than the extensive literature emphasizing its significance has implied. Responses to the two problems often converge to a significant extent on the position that aggregation is impermissible because the intuition to divert is both more weakly held and more subject to revision than the intuition against pushing. We show this convergence when the problems are presented together. Moreover, while the strictures against pushing are typically seen as mandatory (and hold even when following the strictures disadvantages kin or more identified parties), the obligation to divert is rarely unqualifiedly mandatory, and even its permissibility is sensitive to potential harm to kin and identifiable people.