A triad oddity task was used to investigate developmental changes in perceived similarity among animals and humans. Four, five, seven, and eight year-old children and undergraduates were presented with triads consisting of a human, a non-human primate, and a non-primate animal, and asked about taxonomic similarity ("which two are the same kind of thing?") and psychological similarity ("which two think and feel the same way?"). At all age groups, humans were seen as taxonomically unique. Beliefs about psychological similarity underwent marked developmental change, from essentially random guessing to belief that humans were psychologically unique to beliefs that humans were psychologically similar to other primates. There was little evidence of differentiation between psychological and taxonomic similarity among children. Younger children's responses were apparently guided solely by the human-nonhuman dichotomy, whereas older children and undergraduates were also influenced by the category mammal. Results suggest interesting continuities and discontinuities in the development of folk biological thought, and between folk and scientific biology.
KEYWORDS: similarity, folk biology, conceptual development.
One important challenge in the development of any conceptual domain is establishing an ontology, or delineation of the basic entities that exist in that domain and the important relations that hold among those entities (Wellman & Gelman, 1992). In the domain of biology the ontology is particularly richly structured (e.g, Berlin, 1992); with development, understanding of relations among biological kinds becomes increasingly complex and multifaceted. For example, children become increasingly aware of plants' status as living things (Richards & Siegler, 1986), of taxonomic groupings of plants and animals at different levels of hierarchy (Carey, 1985; Coley, Hayes, Lawson, & Moloney, 2004; Coley, Solomon, & Shafto, 2002; Ross, Medin, Coley, & Atran, 2003), and of ecological and causal relations among species that are potentially orthogonal to taxonomic relations (Coley, Vitkin, Seaton, & Yopchick, 2005).
Another important development involves the way in which human beings are incorporated into the folk taxonomy of living things. Clearly, humans are unique among species in many ways; as such, are humans ever incorporated into the broader folk taxonomy? Are humans seen as similar to other species in some ways, and unique in other ways? How do these beliefs change over the course of development? In this paper I examine these questions by focusing on developmental changes in perceived similarity between humans and other animals. I also focus on two related questions: to what degree does the perceived similarity of humans to other animals depend on the nature of similarity being considered, and to what degree is perceived similarity modulated by sensitivity to intermediatelevel animal categories?
Humans' place in folk biological taxonomy
According to geneticists, humans are African great apes. Our closest living relatives are chimpanzees and bonobos, with whom we share a common ancestor that lived c. 5-8 million years ago. Indeed, we are more closely related to chimpanzees and bonobos than either of those are to the gorilla, our next closest primate relative. However, folk biological taxonomies-particularly those found in industrialized western societies-tend to see humans as essentially separate from other species. Indeed, on some accounts, learning that humans are biologically "one animal among many" is a major developmental milestone in the acquisition of biological understanding.
For example, according to the "conceptual change" view first outlined by Susan Carey in her seminal 1985 book (see also Carey, 1995; Carey & Spelke, 1996; Johnson & Carey, 1998) children's understanding of animals is initially embedded in the core domain of intuitive psychology wherein behavior and intention rather than biological process are the central components. …