The effect of time pressure and the spatial integration of the stimulus dimensions on overall similarity categorization

The effect of time pressure and the spatial integration of the stimulus dimensions on overall similarity categorization. Fraser Milton (f.n.milton@ex.ac.uk) School of Psychology, University of Exeter, United Kingdom Lotta Viika (lv222@ex.ac.uk) School of Psychology, University of Exeter, United Kingdom Holly Henderson (hssh201@ex.ac.uk) School of Psychology, University of Exeter, United Kingdom Andy J. Wills (a.j.wills@ex.ac.uk) School of Psychology, University of Exeter, United Kingdom several influential theories of natural categories. The “classical” view postulates that natural categories are made up of necessary and jointly sufficient features (e.g., Bruner, Goodnow, & Austin, 1956). If an item has the necessary feature (or features) it can be considered a member of that category regardless of the rest of its properties. In recent times, there has, however, been a growing emphasis on the idea that natural categories are organized around an overall similarity (family resemblance) structure (Rosch & Mervis, 1975). Under this theory, an item does not have to possess any single feature, but as long as it has enough features that are characteristic of that category it can be considered a member of that group. In an overall similarity structure, within-group similarity is maximized and between-group similarity is minimized. One advantage of a family resemblance strategy is that it enables one to make inferences about other properties of an item (e.g., Lassaline & Murphy, 1996). For instance, by classifying an item as a bird, it enables one to infer that it can fly, has feathers, lays eggs, and sings. Given that the family resemblance theory appears a more compelling account than the classical view, it is surprising that initial studies showed that when people are asked to classify a group of stimuli without any feedback from the experimenter (often known as free classification) they tend to sort by a single dimension (e.g., Ahn & Medin, 1992; Ashby, Queller & Berretty, 1999; Imai & Garner, 1965; Medin, Wattenmaker, & Hampson, 1987). More recent work, however, has shown that overall similarity sorting can be increased by a number of factors including background knowledge (Spalding & Murphy, 1996), stimulus structure (Pothos & Close, 2008), perceptual discriminability (Milton & Wills, 2008), pairwise rather than multiple stimulus comparisons (Regehr & Brooks, 1995), the prior application of an overall similarity sort to different stimuli (Milton & Abstract A free classification study is presented in which the effect of time pressure and the spatial integration of the stimulus dimensions on overall similarity sorting is investigated. A 2 x 2 between-subjects factorial design was employed with the factors being the level of time pressure (high/low) and the spatial integration of the stimulus dimensions (high/low). The results showed, consistent with Milton and Wills (2004), that spatially separable stimuli resulted in a greater level of overall similarity sorting than more spatially integrated dimensions. Furthermore, participants under low time pressure produced a greater level of overall similarity sorting than those under high time pressure, consistent with Milton, Longmore and Wills (2008). Critically, there was also a significant interaction between time pressure and the level of spatial integration, with the integration effect being greater under low time pressure than under high time pressure. These findings provide support for the idea that overall similarity sorting can be the result of an effortful, deliberative process. Keywords: free classification, overall similarity, unidimensional, time pressure, spatial separability. Introduction Categorization is the process by which items encountered in the world are divided into groups of things. Our ability to categorize is so fundamental to our mental life that it is easily overlooked. As an illustration of its importance, in a world of only proper nouns, you could have the concept of “Rover” but not of “Dog”, of “Barrack Obama” but not of “President”. The process of categorization must necessarily be highly constrained due to the virtually infinite number of objects we encounter in our everyday environment. One reasonable assumption is that the categories we prefer to create would reflect the underlying structure of objects we encounter outside the laboratory. Over the years there have been