Kevin Tobia
Yale University
George Newman
Yale University
Joshua Knobe
Yale University
The “Twin Earth” philosophical thought experiment has importantly influenced the study of psychological essentialism. The standard philosophical intuition about that thought experiment suggests that it is an entity’s deeper causal properties—and not its superficial features—that are criterial for categorization. Four studies suggest that people do not share this intuition. Instead, people have two distinct criteria for category membership, one based on superficial features and one based on deeper causal properties. Studies 1a and 1b show that people reject the standard Twin Earth intuition, instead endorsing two (opposing) criteria for category membership. Study 2 shows that contextual cues affect categorization of entities in Twin Earth cases. Studies 3a and 3b extend these findings by looking both to a real-world case involving genetically modified organisms and a population of graduate students from elite universities. Together, these studies provide an enriched understanding of essentialized concepts. People do not endorse the standard Twin Earth intuition, categorizing entities solely on the basis of their deep causal properties; instead, people employ two sets of criteria in natural kind categorization.