Although most previous studies on categorization

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1 Japanese Psychological Research 1987, Vol.29, No.3, Effects of category structure on children's categorization TAKESHI SUGIMURA and TOYOKO INOUE Department of Psychology, Nara University of Education, Takabatake-cho, Nara 630 In two experiments 6-year-olds were given a category-generalization task to assess the prototype formation, the categorization mode, and the correct classification of original exemplars. In the task with DT (Dimension+Typicality) rule, children could categorize the exemplars based on either a criterial dimension or typicality (or overall similarity). In the task with T (Typicality) rule, children could categorize the exemplars based only on typicality. The amount of prototypical values in each category was varied in two tasks. For DT tasks the number of prototypical responses did not change by the amount of typicality, whereas for T tasks it increased with the amount of typicality, Children who could correctly classify the original exemplars tended to form the prototype. The dimensional (or analytic) mode was used more often in DT tasks, whereas the typical (or holistic) mode was used more often in T tasks. It was concluded that prototype formation and categorization mode depend on amount of typicality and rule of categorization. Key words: category structure, prototype formation, categorization mode, dimensional mode, typical mode. Although most previous studies on categorization of artificial stimuli were performed mainly with adult subjects (Homa, 1984; Medin & Smith, 1984; Smith & Medin, 1981 for review), there has been an increasing concern about young children's categorization (Kemler, 1983; Younger & Cohen, 1985 for review). Studies on children's categorization can be roughly classified into two research areas: the mode of categorization and the formation of prototype. The purpose of the present study was to examine the effects of category structure on the prototype formation and the categorization mode, In the present study the category structure was varied by the amount of typicality and the rule of categorization. As for the categorization mode, several researchers have presented evidence that there is a general trend in perceptual and cognitive development from the holistic to the analytic mode (e.g., Kemler, 1983; Shepp, 1983). In the holistic mode, stimuli are treated as undifferentiated wholes and related by overall similarity. In the analytic mode, stimuli are treated as analyzed sets of dimensional components and related by shared dimensional values. Kemler (1983) suggested that the analytic mode is more abstract than the holistic mode and that in the course of normal perceptual and cognitive development the balance shifts from holistic to analytic. To examine the developmental change in categorization mode, Smith (1979) gave a category-generalization task to kindergartners, second graders, and fifth graders. In this task the subject was shown two groups of objects and told that the exemplars in each group belonged together. Then the subject was asked where the new test exemplars went: in the first group, in the second group, or not at all. Two types of category structure were provided. In the condition SIM+DIM, the subject could classify the test exemplars on the basis of either overall similarity or a criterial dimension. In the condition DIM, the subject could classify the test exemplars only on the basis of a criterial dimension. She found that young children generalize a category by overall similarity while older children generalize a category by a cri-

2 Category structure in children's categorization terial dimension. The findings are in line average prototype when dimensional values with the general developmental trend are continuous and difficult to discri- from the holistic to the analytic mode. minate, whereas they tend to form the Kemler Nelson (1984) assumed that the modal prototype when dimensional values categories with a family-resemblance structure are discontinuous and easy to discriminate proposed by Rosch and Mervis (1975) are processed by the holistic mode while (Goldman & Homa, 1977; Neumann, 1977; Strauss, 1979). It is more interesting for us that Strauss (1979) and Younger dimension) are processed by the analytic (1985) found that even 10-month-old infants could form the prototype by using a mode. To test this assumption, kindergartners and fifth graders were given either a task which could be solved on the basis of overall similarity relations in a family- visual habituation-preference paradigm. resemblance structure or a task which could be solved on the basis of a single criterial attribute. The kindergartners learned more easily the family-resemblance task than the criterial attribute task while the fifth graders learned both tasks at about equal ease. The findings suggest that kindergarten children are more sensitive to the family-resemblance structure than the criterial attribute structure. Among several models of the prototype formation, two models have been compared and contrasted in the categorization of artificial objects (e.g., Goldman & Homa, 1977; Neumann, 1977, Solso & McCarthy, 1981; Strauss, 1979). According to the central tendency or feature averaging model, the prototype of a category is composed of the averaging or mean values of attributes varying within the category. According to the attribute frequency or feature count model, on the other hand, the prototype of a category is composed of the most frequently or most commonly experienced values of attributes varying within the category. It is expected, therefore, that if categorization is performed in accordance with the feature averaging model, an average prototype will be formed and if categorization is performed in accordance with the feature count model, a modal prototype will he formed. Although which prototype is formed depends on several factors, it has been generally found that adults tend to form the In the present study an attempt was made to explore whether kindergarten children can form the prototype in a modified category-generalization task. As mentioned above, the categorization mode and the prototype formation have been treated as independent research areas in the previous studies. As an attempt to relate the two research areas in children's categorization, a category-generalization task which can probe simultaneously both the formation of prototype and the mode of categorization was devised in the present study. Our primary concern is to examine (a) whether kindergarten children can form the prototype and (b) if the prototype is formed, what mode of categorization is used. Experiment I In Experiment I DT (Dimension+Typicality) tasks which can categorize on the basis of either a criterial dimension or typicality (or overall similarity) were provided and the amount of typicality was varied at four levels. After the presentation of exemplars of two categories, test exemplars were given to assess the prototype formation, the categorization mode, and the correct classification of original exemplars. The hypotheses to be tested in the present experiment were as follows: (a) if the prototype formation depends on the amount of typicality, the number of prototypical responses will increase with the amount of typicality, (b) if kindergarten children prefer the holistic to the analytic mode, they

3 T. Sugimura and T. Inoue will categorize the test exemplars on the basis of typicality rather than dimensionality, and (c) if kindergarten children are sensitive to the amount of typicality in each category, the categorization based on typicality will increase with the amount of typicality. Subjects. The subjects were 120 kindergarten children (60 males and 60 females) with a mean age of 6: 02 (5: 08-6: 08). They were assigned to four groups of 30 subjects in each matching the mean ages and the number of males and females. Additional eight subjects were discarded from the experiment for strong position preferences. Stimuli. As is illustrated in Fig.1, the stimuli were 32 schematic faces varying with two values in each of five dimensions: size of ears (large vs. small), position of parting hair (left vs. right), separation of eyes (narrow vs. wide), shape of nose (triangle vs. square), and size of mouth (large vs. small). These faces were drawn in 8.5cm white cardboard. In Fig.1, the left face with the notation of has large ears, right position of parting hair, wide separation of eyes, a triangular nose, and a large mouth. The right face with the notation of has small ears, left position of parting hair, narrow separation of eyes, a square nose, and a small mouth. Category structure. The prototypical exemplar in Category 1 is shown by the notation of and has Value 1 in all dimensions. The prototypical exemplar in Category 0 is shown by the notation of and has Value 0 in all dimensions. Five sets with different prototypical exemplars were provided to counterbalance dimensional salience. The criterial dimension was ears in Set A, a mouth in Set B, a nose in Set C, eye separation in Set D, and parting hair in Set E. As is shown in Fig.2, each task had two categories of four exemplars which were derived from the prototypical exemplars of Category 1 and Category 0, respectively. All tasks had the category structure which could categorize on the basis of either a criterial dimension (Dimension a) or typicality (amounts of prototypical values). The amount of typicality was varied from task to task. In Task four prototypical values (1 in Category 1 and 0 in Category 0). Thus each category had a total of 16 prototypical values. In Category 1 Category 0 Fig.1. Examples of stimuli.

4 Task 4443DT, three exemplars in each category had four prototypical values and one exemplar had three prototypical values, which resulted that each category had a total of 15 prototypical values. In Task Category structure in children's categorization 123 had four prototypical values and three exemplars had three prototypical values, which resulted that each category had a total of 13 prototypical values. In Task 3333DT, four exemplars in each category had three prototypical values and each category had a total of 12 prototypical values. Test. The test exemplars to assess the prototype formation and the categorization mode are shown at the bottom of Fig. 2. If the subject classified the prototypical exemplar into Category 1 and the prototypical exemplar into Category 0, he was assumed to make a prototypical response and form a prototype. As for the categorization mode, if the subject classified the test exemplar into Category 1 and the test exemplar into Category 0, he was assumed to categorize on the basis of typicality. If the subject classified the test exemplar into Category 0 and the test exemplar into Category 1, he was assumed to categorize on the basis of dimensionality. If the subject classified the two test exemplars into either one category, the mode of categorization was not determined. In addition to these test exemplars, two test exemplars, one from the original exemplars in Category 1 and the other from the original exemplars in Category 0, were provided for each task to check whether the original exemplars were correctly classified in the test trials. These exemplars are not shown at the bottom of Fig.2 since selected original exemplars differed from task to task and from test to test. Out of four original exemplars in each category, two exemplars were used twice as the test exemplars and the remaining two were used once. If the subject classified the original exemplar of Category 1 into Category 1 and the original exemplar of Category 0 into Category 0, he was asstuned to classify correctly the original exemplars. Procedure. The subjects were tested individually in a room of their kindergarten. When a subject faced the experimenter across a table, two schematic houses drawn cardboard were placed on the table side by side in front of the subject. Each house had a roof and five rectangle windows window with a red frame was placed in the center and was used to put the test exemplars. The other four windows with a black frame were placed around the redframed window and were used to put the original exemplars. The original exemplars were presented in the following way."let's play a grouping game.(the experimenter presents an exemplar in Category 1.) Look at this face carefully. This child is a Kenchan's fellow.(the experimenter put the card in one of the black-framed windows in the right house.) This is a Kenchart's fellow, too." In this way, four exemplars in Category 1 were put in the black-framed windows in the right house. "Well, look at this house.(the experimenter points to the left house and then presents an exemplar in Category 0.) This child is a Takeshichan's fellow.(the experimenter put the card in one of the black-framed windows in the left house.) This child is a Takeshichan's fellow, too." In this way, four exemplars in Category 0 were put in the black-framed windows in the left house. Thus the subject was informed that the exemplars in the right house belong together (Category 1) and that the exemplars in the left house belong together (Category 0). Immediately after the presentation of original exemplars, the test exemplars were given in the following way."this time, I give you several cards one by one. Some children belong to Kenchan's fellows.(the

5 experimenter points to Kenchan's house.) Some other children belong to Takeshichan's fellows. (The experimenter points to Takeshichan's house.) If you think a child belongs to Kenchan's fellows, put the card in the red-framed window of Kenchan's house.(the experimenter points to the red-framed window.) If you think a child belongs to Takeshichan's fellows, put the card in the red-framed window of Takeshichan's house.(the experimenter points to the red-framed window.) Now, does this child belong to Kenchan's fellows or Takeshichan's fellows?" Then the six test exemplars were given in a random order at the subject's pace. Immediately after the subject put a card in the window, the experimenter removed the card and presents the next card with the instruction," Now, does this child belong to either fellows?" No feedback was given to the subject's responses. After the six test exemplars were given, the eight original exemplars were removed from the windows. Again the original exemplars were presented and then the test exemplars were given. In this way, the original and the test exemplars were presented repeatedly six times. In the three sessions with odd numbers, the presentation of Kenchan's fellows was followed by Takeshichan's fellows and the order of the presentation was reversed in the remaining three sessions. Results The number of trials which could correctly classify the original exemplars in the test trials was counted for four tasks and an analysis of variance was performed. The result showed no significant task difference. The total mean number of trials In the present study the formation of prototype was assessed in terms of the prototypical responses in the test trials. Table responses in the test trials. To check whether the prototype was well formed or not, t-tests were performed for the differences between each of the total means and the chancc mean (1.5). The chance mean was calculated by the following way. As the prototypical response in each trial was one of the four possible response patterns and the probability of its occurrence was.25 in each trial, the chance mean sults showed that the number of prototypical responses exceeded significantly the ance showed that only the main effect of mode is highly significant, F(2, 232)= between the typical and the undetermined.001, but the difference between the dimensional and the typical mode was not significant, t(232)=1.61, p>.10. When the four tasks were pooled, the proportion of each mean value to the total mean value (4.26) was 48.1% for the dimensional mode, 37.3% for the typical mode, and Table 1 Means and SDs of prototypical responses (Exp. I)

6 Category structure in children's categorization Correlation coefficients between correct classifications of original exemplars and prototypical responses (Exp. 1) 14.6% for the undetermined mode. Therefore, most subjects classified the test exemplars mainly on the basis of dimensional and typical modes. As is shown in Table 1, the sample values showed that the means of dimensional mode are larger for the tasks with smaller amounts of typicality than for the tasks with larger amounts of typicality, while the means of typical mode are larger for the tasks with larger amounts of typicality than for the tasks with smaller amounts of typical) analysis of variance yielded, however, that the expected interaction was not statistically significant, F(3, 116)=1.25, Table 2 shows correlation coefficients between the correct classifications of original exemplars and the prototypical responses. The coefficients for the total prototypical responses and the dimensional mode were positive and significant in all tasks. This suggests that the subjects who could correctly classify the original exemplars in the test trials tended to form the prototype and use the dimensional mode. Except for Task 4333DT the coefficients for the typical and the undetermined modes were negative but not significant. Discussion The findings that the number of total prototypical responses exceeded significantly the chance mean present clear evidence that kindergarten children can also studies with adults and infants. Whether or not kindergarten children can form prototypes in the tasks other than the category-generalization task used in the present study remains for further researches. The prototype formed in the present experiment seems to be the modal prototype rather than the average one because it is composed of the most frequently experienced values in each category. Although the sample means tended to increase with the amount of typicality (see Table 1), the hypothesis that the number of prototypical responses increases with the amount of typicality was not statistically confirmed. The finding that there was no significant effect of the amount of typicality on the prototype formation may be attributed to the type of category structure used in the present experiment. Since the present category structure had the categorization rule which can categorize on the basis of either a criterial dimension or typicality, the subjects could attend to the criterial dimension rather than the amount of typicality and the typicality effect might be weakened. If so, it is assumed that the typicality effect will be observed when the subjects are given the tasks which can categorize only on the basis of typicality. This assumption will be examined in Experiment II. Our second hypothesis that the subjects categorize the test exemplars on the basis of typicality rather than dimensionality

7 was not also confirmed. As is shown in Table 1, the subjects categorized the test exemplars more frequently (though not significant) by using the dimensional mode than by using the typical mode. Differing from the inference from the general trend in perceptual and cognitive development, the present finding suggests that kindergarten children tend to prefer the analytic to the holistic mode. If the category structure which is easy to categorize by the holistic mode is provided, it will be expected that the subjects can categorize the test exemplars more frequently by using the typical mode than by using the dimensional one. To test this expectation, the tasks which can process by the holistic mode will be devised and compared with the findings in Experiment I. Children's preferred mode may depend on stimuli and tasks for assessing the categorization mode. As for the stimuli, a direct comparison is impossible between the stimuli in the present and the Smith (1979) studies since Smith used isosceles triangles of equal area which varied in height and color. Although the schematic faces were used in the present and the Kentler Nelson (1984) studies, an inspection of both faces suggests that the present faces are more distinctive in the dimensional values than the Kemler's faces. It is reasonable to consider, therefore, that kindergarten children are easy to attend to the dimensional values and to use the analytic mode when the dimensions are salient or distinctive. The dimensional salience must be an important factor affecting the categorization mode. As for the tasks, the present experiment assessed the mode by the test performance on the task which had two categorization rules, whereas the Kemler Nelson study assessed the mode by comparing the learning rate on the two tasks which had different categorization rules. Therefore, the difference in the tasks for assessing the categorization mode may cause the difference in the mode of categorization. Since the task by mode interaction was not significant, our third hypothesis that the categorization based on typicality increases with the amount of typicality was not confirmed. The finding that there was no significant typicality effect may he also attributed to the type of category structure. As mentioned earlier, the sample values tended to show that the subjects used more frequently the dimensional mode for Tasks 4333DT and 3333DT than for Tasks 4444DT and 4443DT and that they used more frequently the typical mode for Tasks 4444DT and 4443DT than for Tasks 4333DT and 3333DT. If the tasks which can categorize only on the basis of typicality are given, therefore, it is expected that the effect of the amount of typicality will become more evident. This expectation will be examined in Experiment II. In previous studies (e.g., Goldman & Hoina, 1977; Hintzman & Ludlam, 1980; Hoina & Little, 1985; Posner & Keele, 1968), the correct classifications of original exemplars were treated independently and the mean values of both measures were compared. Rather than comparing the mean values, the correlation coefficients of both measures were calculated in the present study. An important finding in the present experiment was that the subjects who could correctly classify the original exemplars in the test trials tended to form the prototype and use the dimensional mode but not to use the typical one. To our knowledge, such relationships have not been reported in the previous studies. When the tasks which can categorize only on the basis of typicality are given, what relations are obtained between the two measures? This question will be answered in Experiment II. Experiment II In Experiment II T (Typicality) tasks which can be categorized on the basis of typicality only were provided and the

8 amount of typicality was varied at two levels (4443 and 4333). Performances on these new tasks were compared with those on Tasks 4443DT and 4333DI' in Experiment I which had the same amounts of typicality. As was suggested earlier, it was hypothesized that (a) in T tasks the number of prototypical responses will increase with the amount of typicality,(b) the dimensional mode will be used more frequently in DT tasks than in T tasks, whereas the typical mode will be used more frequently in T tasks than in DT tasks, and (c) the categorization based on typicality will increase with the amount of typicality. Method Category structure in children's categorization first factor was the amount of typicality (4443 vs. 4333) and the second factor was the rule of categorization (DT vs. T). The subjects were 120 kindergarten children (60 males and 60 females) with II. a mean of 6: 02 (5: 08-6: 09). They were assigned to four groups of 30 subjects in each matching the mean ages and the number of males and females. Additional seven subjects were discarded from the experiment for strong position preferences. Out of 120 subjects 60 were the same ones used in Experiment I. The stimuli were identical to those used in Experiment I. As is shown in Fig. 3, four tasks with different category structures were provided. Task 4443DT and Task 4333DT were identical to those used in Experiment I. These tasks could be categorized on the basis of either the criterial dimension (Dimension a) or typicality (amounts of prototypical values). New two tasks, Task 4443T and Task 4333T, were introduced in Experiment II. These tasks could be categorized only on the basis of typicality because there was no criterial dimension which had only Value 1 or Value 0. In Task 4443T, three exemplars in each category had four prototypical values and one exemplar had three Fig. 3. Category structure used in Experiment prototypical values. Thus each category had a total of 15 prototypical values. In Task 4333T, one exemplar in each category had four prototypical values and three exemplars had three prototypical values. Thus each category had a total of 13 prototypical values. The test exemplars and experimental procedure were identical to those in Experiment I. Results For Tasks 4443DT and 4333DT, the resnlts nhtained in Experiment I were used analysis of variance for the number of correct classifications of the original exemplars revealed that only the main effect of rule was significant, F(1, 116)=11.06, p<.01, MSe=1.22. The number of correct classifications was greater for two DT tasks (X=3.77) than for two T tasks (X= 2.82), which suggests that the subjects tended to classify the original exemplars by using the dimensional values.

9 T. Sugimura and T. Inoue Table 3 Means and SDs of prototypical responses (Exp. II) Table 4 Correlation coefficients between correct classifications of original exemplars and prototypical responses (Exp. II) Table 3 shows means and.sdsof prototypical responses in the test trials. A 2 variance was performed. Themaineffect of amount was slightly significant, F(1, ber of prototypical responses was greater for two 4443 tasks (X=4.40) than for two 4333 tasks (X=3.80). Although the interaction between amount and rule was not significant, the sample means suggest that the amount effect is mainly attributed to two T tasks rather than two DT tasks. In fact, the difference between Task 4443T and Task 4333T was significant,t(58)= 2.13, p<.05, whereas the difference between Task 4443DT and 4333DT was not significant, t(58)=0.61, p>.10. The main effect of mode was also significant, F(2, 232)=11.34, p<.01, MSe= The difference between the dimensional and the undetermined modes was significant, t(232)=3.23, p<.01, but the difference between the dimensional and the typicalmodes was not significant, between rule and mode was significant, F(2, 232)=7.25, p<.01, several t-tests were performed to test the simple effects. (a) The dimensional mode was used more frequently in two DT tasks (X=2.15) than in two T tasks (X=0.90), t(348)=3.98, p<.01, whereas the typical mode was used more frequently in two T tasks (X=2.20) than in two DT tasks (X=1.57), t(348)= 2.01, p<.05.(b) Although the difference between the two modes was not significant in two DT tasks, 4232)=0.63, p>.10, the typical mode was used more frequently in two T tasks than the dimensional mode, The interaction between amount and mode was slightly significant, F(2, 232)= 2.86, p<.10, MSe=3.96, which showed that although there was no significant difference for the dimensional mode (X=1.50 for two 4443 tasks and X=1.55 for two 4333 tasks), the typical mode was used more frequently in two 4443 tasks (X= 2.34) than in two 4333 tasks (X=1.43),

10 Category structure in children's categorization 129 t(348)=2.90, p<.01. Table 4 shows correlation coefficients between the correct classifications of original exemplars and the prototypical responses. The correlation coefficients in the total prototypical responses were significantly positive for all tasks. This suggests that in T tasks as well as in DT tasks, the subjects who could correctly classify the original exemplars tended to form the prototype. The coefficients for the dimensional mode were significantly positive in two DT tasks. For the typical mode, a significantly positive coefficient was obtained in Task 4443T while a significantly negative coefficient was obtained in Task 4333DT. The subjects who could correctly classify the original exemplars tended to use the typical mode in Task 4443T but not to use it in Task 4333DT. Discussion In T tasks the finding that the number of prototypical responses increased with the amount of typicality supports our first hypothesis. Although the category structures of Task 4443T and Task 4333T differed only in two prototypical values (15 vs. 13), the subjects were very sensitive to this slight difference in the prototypical values. This fact suggests that kindergarten children can attend to overall similarity relations when tasks have the categorization rule which can categorize exemplars only on the basis of typicality. In DT tasks which can categorize on the basis of either a criterial dimension or typicality, however, the subjects were not so sensitive to the amount of typicality and not so attentive to overall similarity relations. It can be concluded, therefore, that the typicality effect on kindergartners' categorization depends on the category structure. Our second hypothesis was confirmed by the significant interaction between rule and mode. The dimensional mode was used more frequently in DT tasks than in T tasks, whereas the typical mode was used more frequently in T tasks than in DT tasks. In general, when exemplars can be categorized on the basis of either a criterial dimension or overall similarity, the exemplars are processed by the analytic mode rather than by the holistic one. When exemplars can be categorized only on the basis of overall similarity, on the other hand, the exemplars are processed by the holistic mode rather than by the analytic one. It is suggested, therefore, that whether or not exemplars are processed in terms of the holistic or the analytic mode depends on category structure as well as age. Although the previous studies concerned mainly the developmental trend from the holistic to the analytic mode, further researches are needed to examine the interactive effects of category structure and age on the categorization mode. The finding that the typical mode was used more frequently in two 4443 tasks than in two 4333 tasks supports our third hypothesis. Since there was no difference in the dimensional mode between two 4443 and two 4333 tasks, it is concluded that the amount of prototypical values affects exclusively use of the typical mode. The sample means suggests that the typicality effect was greater for T tasks than for DT tasks. In fact, the difference between Task 4443T and Task 4333T was significant, t(58)=2.17, p<.05, but the difference between Task 4443DT and Task 4333DT was not significant, t(58)=1.40, p>.10. Although the correlation coefficients for the total prototypical responses were significantly positive in all tasks, those for the dimensional and the typical modes showed very different patterns depending on the categorization rule and the amount of typicality. Especially when the tasks had larger amounts of typicality (4443), the subjects who could correctly classify the original exemplars tended to use the dimensional mode in DT task and the typical mode in T task. From these correla-

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