ABSTRACT
At least some animals share with humans the ability to react differently to stimuli that belong to different categories. In a study by Herrnstein, Loveland, and Cable (1976), for instance, pigeons were trained to discriminate between photographic slides on the basis of whether or not they contained a picture of a tree. Once this discrimination had been learned, it transferred readily to novel slides. This outcome has been shown to have considerable generality. It has been revealed with such diverse categories as water, a specific person, fish, cars, letters of the alphabet, chairs, and even a cartoon character—Charlie Brown (e.g., Bhatt, Wasserman, Reynolds, & Knauss, 1988; Cerella, 1980; Herrnstein et al. 1976; Herrnstein & de Villiers, 1980; Morgan, Fitch, Holman & Lea, 1976). Moreover, the ability to categorize is not confined to pigeons, it has also been shown with monkeys (D'Amato & Van Sant, 1988; Roberts & Mazmanian, 1988; Schrier, Angarella, & Povar, 1984; Schrier & Brady, 1987), quail (Kluender, Diehl, & Killeen, 1987), chickens (Ryan, 1982), and a parrot (Pepperberg, 1983).
