Previous studies have found that an object can be categorized faster at a basic level (hammer) than at either a subordinate (club hammer) or a superordinate level (tool). While some attribute this result to basic categories having more distinctive attributes, other factors might cause this result. For example, basic categories routinely have shorter and more frequent names than do subordinates and are typically learned earlier and occur more often than either subordinate or superordinate categories. To determine why objects are categorized fastest at the basic level, three experiments were conducted, all of which used artifical subordinate, basic, and superordinate categories, and all of which either held constant or systematically varied several of these "other" factors. All three studies supported the finding that objects can be categorized fastest at the basic level (but the relative speeds of subordinate and superordinate categorizations differed from past results); and all three strongly supported the claim that distinctive attributes are the factor underlying the results, though it appears that only perceptual attributes are critical. (Author/HOD)