Inferences About Personal Identity

Inferences About Personal Identity Sergey Blok ( s-blok@northwestern.edu ) George Newman ( g-newman@northwestern.edu ) Jennifer Behr ( jennifer.behr@yale.edu ) Lance J. Rips ( rips@northwestern.edu ) Department of Psychology, Northwestern University, Evanston, Illinois 60208 USA Abstract Features of Person Identity We investigate the features people use in making inferences about continuity of individual persons. Using a transforma- tion paradigm, we show that people weigh both continuity of the brain and continuity of mental content. Across experi- ments, we document instances in which participants are more likely to assert individual continuity than continuity of per- sonhood. We discuss these results in terms of a hierarchical view of concepts and philosophical work on personal identity. Introduction People are sensitive to the effects that transformations have on membership in basic-level categories (e.g., Gelman & Wellman, 1991; Keil, 1989; Rips, 1989). For example, Rips (1989) asked participants to read stories about creatures of one category (e.g., birds) that came to resemble those of another (e.g., insects). If the transformation was due to ac- cidental factors, participants believed that the creature re- mained a member of the original category. If the transformation was part of normal maturation, however, participants judged the creature a member of the second category. In general, transformations that alter an object’s core properties also tend to change the object’s category, while transformations that alter an object’s surface or incidental properties tend to preserve its category. Despite the relatively large number of studies that address questions of category membership continuity, there have been few studies addressing reasoning about individual con- tinuity (see Hall, 1998, Johnson, 1990 and Liittschwager, 1994, for exceptions in the developmental literature). The central question is how we decide that a particular individ- ual – for example, my dog Fido – is the same individual (still Fido) across transformations. This question is poten- tially different from the one about membership – whether this individual is still a dog. We investigate here two issues concerning reasoning about individuals. First, we explore the kinds of features people use in judging continuity of identity. Second, we contrast the ways in which people reason about class mem- bership and about identity continuity. What properties does a person at time t 1 need to share with one at time t 2 in order for that individual to be the same at both temporal markers? In making such judgments, people may be phenomenalists, relying on continuity of appear- ance. In a preliminary experiment, we created stories that varied the type of transformation that a hypothetical target person undergoes. One set of participants -- the Plastic Surgery group -- read a scenario about Jim, a male accountant, who receives plastic surgery to alter his appearance cosmetically to resemble that of Marsha, a female actress. Another set of participants -- the Brain Transplant group -- read a similar story in which Jim’s brain is replaced with that of Marsha. After reading the story, both groups supplied judgments of Jim’s identity change – whether the individual was still Jim or had become Marsha after surgery. Results indicated that a greater proportion of participants in the Brain Transplant group believed Jim’s identity had changed than in the Plas- tic Surgery group (45% and 15%, respectively, 2 (1, 39) = 4.29, p < . 05). These results suggest that changes in appearance are ordi- narily not enough to warrant change in identity. The finding parallels earlier studies of natural kinds that show that peo- ple tend to reject mere appearance as evidence for category membership when appearance conflicts with deeper proper- ties of the category in question. It has often been suggested that natural kinds’ hidden, causally central properties are used for categorization and induction (Ahn, 1998; Gelman & Hirschfeld, 1999), while surface features such as appear- ance are used for similarity judgments (Keil, 1989; Rips, 1989). But although our results hint that our participants are not folk-phenomenalists, they leave open the question of what criteria they do use to assess personal identity. The question of criteria for identity is one of the oldest in metaphysics. Writers on the physicalist side (e.g., Aristotle, Wiggins) argue that continuity of the body, or, more impor- tantly, the brain, is critical to identity. According to this view, a person P 2 at time t 2 is the same person as P 1 at t 1 if