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Preference for peers in infancy
Infant Behavior & Development 29 (2006) 584–593
Preference for peers in infancy Wakako Sanefuji a,b,∗ , Hidehiro Ohgami c , Kazuhide Hashiya c a
Graduate School of Human-Environment Studies, Kyushu University, 6-19-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan b Japan Society for the Promotion of Science Research Fellow c Faculty of Human-Environment Studies, Kyushu University, Fukuoka, Japan Received 8 March 2006; received in revised form 17 June 2006; accepted 28 July 2006
Abstract Previous findings suggest that infants prefer other infants from among individuals of various ages. There are two explanations for this: one is that like adults, infants prefer babyish characteristics; the other is that infants prefer individuals who are similar to themselves. We examined whether infants respond differently to same-age infants from among older or younger infants. This might allow the possibility of methodologically separating the two explanations for peer preference in infancy. The results revealed that 9-month-olds showed peer preference when they saw movies of infants. Further experiments showed that 6- and 9-month-olds preferred static images of same-age infants. The lack of preference of 6-month-olds for movies of same-age infants could be related to their ability to process information. Thus, the cue for peer preference may be physical similarity to oneself, rather than the strength of babyish characteristics reflecting a particular developmental stage. © 2006 Elsevier Inc. All rights reserved. Keywords: Infants; Preference; Peers; Similarity; Physical appearance
When infants encounter other individuals, they tend to look at their peers (age mates) eagerly. Infants looked longer at photos, drawings, and movies of infants than at those of older children (Lewis & Brooks, 1975; McCall & Kennedy, 1980; Sanefuji, Ohgami, & Hashiya, 2005). In another study, when a child approached an infant, the infant smiled; however, the opposite occurred when a midget or an adult approached the infant (Brooks & Lewis, 1976). Infants expressed more positive affect, for instance, they smiled, vocalized, and gestured (e.g., banged their arms) more to pictures and movies of infants than to those of older children or adults (Lewis & Brooks, 1975; Sanefuji et al., 2005). All these findings suggest that infants respond differently to individuals of various ages and that they prefer other infants to older children. However, the basis of such peer preference is still under discussion. One explanation for this tendency is the preference for babyish characteristics. Adults tend to exhibit maternal and protective behaviors toward babies. Such behavior may be released by the baby schemas, which refer to infantile characteristics such as the eyes being set at and below the midline of the face, a relatively large head, and clumsy movements (Eibl-Eibesfeldt, 1975; Lorenz (1943) as cited in Shaw & Bransford, 1977). Further, the combination of these infantile characteristics determines the strength of the baby schema. It is suggested that baby schemas are most remarkable just before 1 year of age; studies on adults’ perception of cuteness in human photographs have indicated that infants just below 1 year were selected as being the ∗
Corresponding author. Tel.: +81 92 642 4443; fax: +81 92 642 4443. E-mail address: [email protected] (W. Sanefuji).
0163-6383/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.infbeh.2006.07.007
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cutest (e.g., Hildebrandt & Fitzgerald, 1979; Negayama, 1997). The increase in cuteness at around 1 year of age is likely to be adaptive as a releaser for caretaking behavior, which is considered to be most necessary for infants who have just begun walking independently. Kramer, Zebrowitz, Giovanni, and Sherak (1995) reported that infants as well as adults tended to look longer at a monitor showing an 18-year-old, baby-faced girl than that showing a mature-faced girl of the same age, who was as attractive. They interpreted this result as evidence of the emergence of the preference for baby schemas during infancy. This interpretation should lead to the prediction that just as in the case of adults, from among infants of various ages, infants prefer those that have the strongest babyish characteristics, that is, infants who are around 1 of age. Another interpretation is infants’ preference for others who are similar to themselves. Infants showed a looking preference for photographs and films of other same-sex infants (Aitken (1977) as cited in Bower, 1989; Lewis & Brooks, 1974). On presenting of patch-light displays of a boy and girl infant (biological motion) on film, it was found that the first-fixation measure indicated infants’ significant preference for other same-sex infants (Kujawski & Bower, 1993). Previous studies interpreted their findings as evidence of the ability to identify those infants who share the same gender category (in the sense that they are similar to the infants themselves), not only from feature cues but also from bodily movements. A similar argument has been put forth in imitation studies; infants were capable of representing different types of sensory information on the basis of their ability to match their own behaviors with gestures they observed others performing (Meltzoff & Moore, 1977, 1983, 1995). It is also possible that infants prefer similar-age infants, and not just same-sex infants, from among infants of various ages. Although the two explanations for peer preference are not mutually exclusive, clear evidence regarding which of the two interpretations is more plausible has not been obtained. We attempted to approach this question by investigating infants’ responses to infants of various ages, and not to other individuals of various ages. Thus, the present study examined infants’ responses to the images of infants of various ages displayed on a monitor. Infants with a preference for baby schemas (the baby schemas being judged by adults), irrespective of their ages, are predicted to prefer infants of a particular age with strong babyish characteristics, such as an infant who is approximately 1 year of age (e.g., Hildebrandt & Fitzgerald, 1979; Negayama, 1997). On the other hand, infants with a preference for those similar to themselves are predicted to prefer same-age infants. 1. Experiment 1 In order to investigate which of the two explanations for infants’ preference for other infants is more plausible, Experiment 1 examined infants’ responses to the images of infants of various ages displayed on a monitor. 1.1. Method 1.1.1. Participants Forty infants participated in this experiment: twenty 6-month-olds (12 boys, 8 girls; M = 6.10 months, S.D. = 0.84 months) and twenty 9-month-olds (10 boys, 10 girls; M = 9.16 months, S.D. = 0.78 months). None of the participants had a twin and none of them attended nursery school. Thus, the visual experience of seeing other infants in their daily lives was controlled. The data pertaining to four additional infants were eliminated from the analysis due to the infants’ fussiness. Informed consent was obtained from the caretakers of all the participants. 1.1.2. Stimuli Twelve infants (four 6-, 9-, and 12-month-olds; two boys and two girls for each age group) participated in this experiment as models for the stimuli. All the models were unfamiliar to the participants. Further, all the models were requested to wear identically designed T-shirts and pants; they were provided with three objects (a ball and two toys) to handle (one object was given to the infants at a time). To separate the possibility that a specific way of manipulating an object might influence the infants’ preferential responses, three different objects were provided to the models as stimuli. They handled one object per scene freely against the same background, which implies that 12 types of movies (4 models × 3 objects) were prepared for each age group. These recorded movies were edited to a 20-s movie clip, which had no sound stimuli. Informed consent was obtained from the caretakers of all the models.
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1.1.3. Apparatus The experiment was conducted in a compartment (approximately 155 cm × 110 cm × 155 cm) located in a quiet room, the walls and ceiling of which were covered with a beige cloth, thereby creating a homogeneous background in order to prevent any visual distraction. The monitor (SHARP, LC-20V20, 30 cm × 40 cm), on which the stimuli were displayed, was set on the floor in the compartment. Behind the monitor, a digital video camera was placed to record the responses of the participants during the test session. 1.1.4. Procedure The caretakers sat on the floor at a distance of one meter from the monitor and placed the infants on their lap, using a cuddle seat to set the infants’ position. During the entire test session, the caretaker was requested not to do anything and to move as little as possible. The infants were shown the stimuli of the three different ages one at a time. The trials did not end as soon as the infants looked away; each stimulus was demonstrated for 20 s. The sex of the infant in a stimuli set was the same for each participant; however, the presentation of the stimuli sets depicting infants of either sex was counterbalanced across the participants. The order of the stimuli presentation was also counterbalanced across the participants. 1.1.5. Measurements Using the video camera recordings, we measured the infants’ looking time (LT) and the frequency of banging arms while they looked at the stimuli (BA). LT was defined as the total fixation time for a particular stimulus. Fixation time was measured by an infant’s head orientation. BA was defined as the rhythmical behavior of banging arms largely in the vertical direction. Several kinds of rhythmical behaviors including banging are commonly observed in human infants. Banging stands out for the frequency with which it is observed in children and infants appear to
Fig. 1. The LT in the experiment that used movies as stimuli. Each bar indicates the S.E.s. The upper panel shows the results obtained from the 6-month-old group and the lower panel shows the results obtained from the 9-month-old group.
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be pleasurable in performing this action (Thelen, 1981a). Moreover, a wide variety of relatively non-specific stimuli and internal states appear to facilitate its release (Thelen, 1981b). On the basis of ethological reports (Thelen, 1981a, 1981b), we focused on arm banging behavior, the occurrence of which is largely equal across participants of various ages. Recent studies have begun to use this behavior as a means for measurement in infancy (e.g., Nakata & Trehub, 2004; Sanefuji et al., 2005). In the present study, arm banging served as an indicator of the release of infants’ positive emotion. The relative frequency of BA after controlling the infants’ LT was used in the analysis. As a reliability check, two observers who were blind to the experimental design independently coded 25% of all the data in each experiment. The coding agreements between the two observers were 96.8% for LT and 97.7% for BA. 1.2. Results and discussion The LTs for the 6-, 9-, and 12-month-old infant stimuli were 17.73 s (S.D. = 2.57 s), 17.85 s (S.D. = 2.83 s), and 16.25 s (S.D. = 4.18 s), respectively, for the 6-month-old group, and 17.00 s (S.D. = 2.51 s), 18.35 s (S.D. = 2.13 s), and 16.33 s (S.D. = 2.67 s), respectively, for the 9-month-old group (Fig. 1). An analysis of variance (ANOVA) revealed no significant main effect of the stimuli for the 6-month-old group (F(2, 38) = 2.26, ns). On the other hand, a main effect of the stimuli was observed for the 9-month-old group (F(2, 38) = 3.69, p = 0.034). Nine-month-old infants looked longer at 9-month-old infant stimuli than at 12-month-old infant stimuli (Ryan’s method; t = 2.68, p = 0.01). The relative frequencies of BA while looking at the 6-, 9-, and 12-month-old infant stimuli were 0.90, 0.24, and 0.67, respectively, for the 6-month-old group, and 0.35, 0.82, and 0.18, respectively, for the 9-month-old group (Fig. 2). The ANOVA showed no significant main effect of the stimuli for the 6-month-old group (F(2, 38) = 2.46, ns). On the other hand, for the 9-month-old group, a main effect of the stimuli was observed (F(2, 38) = 3.33, p = 0.046). Nine-month-old
Fig. 2. The relative frequency of BA in the experiment that used movies as stimuli. Each bar indicates the S.E.s. The upper panel shows the results obtained from the 6-month-old group and the lower panel shows the results obtained from the 9-month-old group.
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infants banged their arms more frequently while looking at 9-month-old infant stimuli than at 12-month-old infant stimuli (Ryan’s method; t = 2.45, p = 0.019). On considering the results collectively, it can be said that 9-month-old infants looked longer at 9-month-old infants than at 12-month-old infants. Nine-month-old infants also manifested more behavioral responses while looking at 9month-old infants than at 12-month-old infants who are suggested to have strong babyish characteristics. These results indicate that 9-month-old infants might prefer others who are similar to themselves. On the other hand, 6-month-old infants did not show different response with regard to LT or behavioral responses. There are two possibilities to explain the lack of preferences in 6-month-old infants; one is that a preference for others who are similar to oneself might emerge during the latter period of the first year of life. The other possibility is that 6-month-old infants are unable to deal appropriately with movies that include several types of information such as that pertaining to physical appearance and visual activity. If the first possibility holds true, 6-month-old infants would not show their preferences when any types of information is presented; on the other hand, if the second possibility holds true, 6-month-old infants would show their preferences when other types of information presented. Subsequent experiments were conducted to explore the cause of the lack of preference in 6-month-old infants and the key factor responsible for the preference in 9-month-old infants. 2. Experiment 2 Experiment 2 was conducted to examine the possibility that the physical appearance of the stimuli worked as a cue for infants’ peer preference. 2.1. Method 2.1.1. Participants Thirty-five infants participated in this experiment; nineteen 6-month-olds (10 boys, 9 girls; M = 6.60 months, S.D. = 0.74 months) and sixteen 9-month-olds (8 boys, 8 girls; M = 9.20 months, S.D. = 0.77 months). As in the case of Experiment 1, the daily experience of seeing other infants was controlled among the participants. The data pertaining to seven additional infants were eliminated from the analysis due to the infants’ fussiness. Informed consent was obtained from the caretakers of all the participants. 2.1.2. Stimuli The stimuli used in Experiment 1 were transformed into colored pictures. Due to this, the model’s physical appearance in the original stimulus could be viewed, whereas the relative activity for each original stimulus became impossible to detect. Although more than one picture was prepared from the movies of all the 6-, 9-, and 12-month-old infant stimuli, only one picture of each infant was used. Every picture was displayed for 15 s to sustain the participants’ attention until the end of the presentation. 2.1.3. Apparatus The apparatus used in Experiment 2 was the same as that used in Experiment 1. 2.1.4. Procedure The procedure followed in Experiment 2 was the same as that followed in Experiment 1. 2.1.5. Measurements The measurements used in Experiment 2 were the same as those used in Experiment 1. The coding agreements were 98.1% for LT and 97.2% for BA. 2.2. Results and discussion The LTs for 6-, 9-, and 12-month-old infant stimuli were 9.66 s (S.D. = 3.55 s), 9.82 s (S.D. = 2.91 s), and 9.42 s (S.D. = 3.69 s), respectively, for the 6-month-old infant group, and 10.25 s (S.D. = 3.25 s), 11.91 s (S.D. = 2.49 s), and 9.53 s (S.D. = 3.49 s), respectively, for the 9-month-old infant group (Fig. 3). The ANOVA revealed no
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Fig. 3. The LT in the experiment that used colored pictures as stimuli. Each bar indicates the S.E.s. The upper panel shows the results obtained from the 6-month-old group and the lower panel shows the results obtained from the 9-month-old group.
significant main effect of the stimuli for the 6-month-old group (F(2, 36) = 0.22, ns). On the other hand, a main effect of the stimuli was observed for the 9-month-old group (F(2, 30) = 4.17, p = 0.025). Nine-month-old infants looked longer at 9-month-old infant stimuli than at 6-month-old infant stimuli (Ryan’s method; t = 2.85, p = 0.007). The relative frequencies of BA for 6-, 9-, and 12-month-old infant stimuli were 0.84, 0.25, and 0.17, respectively, for the 6-month-old group, and 0.07, 0.47, and 0.03, respectively, for the 9-month-old group (Fig. 4). The ANOVA was conducted with the stimuli as the independent variable in each group. For the 6-month-old group, a main effect of the stimuli was observed (F(2, 36) = 5.16, p = 0.01). Six-month-old infants banged their arms more frequently while looking at 6-month-old infant stimuli than at 9- or 12-month-old infant stimuli (Ryan’s method; t = 2.44, p = 0.019, and t = 3.03, p = 0.004, respectively). For the 9-month-old group, the stimuli had a main effect (F(2, 30) = 4.01, p = 0.028). Nine-month-old infants banged their arms more frequently while looking at 9-month-old infant stimuli than while looking at 6- or 12-month-old infant stimuli (Ryan’s method; t = 2.37, p = 0.024, and t = 2.53, p = 0.017, respectively). These results suggested that when presented with only physical displays of infant stimuli, both 6- and 9-month-old infants exhibited a preference for looking at same-age infants. This suggests that physical appearance is one of the key factors responsible for 9-month-old infants’ preference for the movies of the same-age infants. However, with regard to the responses of the 6-month-old infants, there appeared to be an inconsistency; 6-month-old infants did not look significantly longer at any stimuli; however, they manifested more behavioral responses to the pictures of same-age infants. Previous studies have suggested that the duration of LT can cover a multitude of looking patterns and this may prevent consideration of important aspects of fixation (Kujawski & Bower, 1993). It is possible that BA can measure some important aspects of fixation that LT is incapable of measuring completely, especially in preverbal period. Additional behavioral responses in 6-month-old infants suggested that they were able to deal appropriately with fewer types of information such as that pertaining to physical appearances and that they show a preference for
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Fig. 4. The relative frequency of BA in the experiment that used colored pictures as stimuli. Each bar indicates the S.E.s. The upper panel shows the results obtained from the 6-month-old group and the lower panel shows the results obtained from the 9-month-old group.
same-age infants. Thus, physical appearances must be important in detecting similarity when 6- and 9-month-old infants compare themselves with other infants. 3. Experiment 3 Experiment 3 was conducted to examine the possibility that the visual activity of the particular stimulus worked as a cue for infants’ peer preference. 3.1. Method 3.1.1. Participants Thirty-five infants participated in this experiment; twenty 6-month-olds (11 boys, 9 girls; M = 6.52 months, S.D. = 0.80 months) and fifteen 9-month-olds (7 boys, 8 girls; M = 9.25 months, S.D. = 0.81 months). As in the case of Experiment 1, the daily experience of seeing other infants was controlled among the participants. The data pertaining to eight additional infants were eliminated from the analysis due to the infants’ fussiness. Informed consent was obtained from the caretakers of all the participants. 3.1.2. Stimuli The stimuli used in Experiment 1 were transformed into movies with mosaic patterns by using a digital AV mixer (Panasonic, WJ-AVE55). Due to this, the relative activity for each original stimulus was represented as dynamic patterns of blinking squires that changed with regard to color and brightness, whereas the physical appearance of the original stimulus was distorted and became impossible to detect. The participants were shown these movies, which were without any sound, for 20 s. 3.1.3. Apparatus The apparatus used in Experiment 3 was the same as that used in Experiment 1.
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3.1.4. Procedure The procedure followed in Experiment 3 was the same as that followed in Experiment 1. 3.1.5. Measurements The measurements used in Experiment 3 were the same as those used in Experiment 1. The coding agreements were 95.3% for LT and 95.6% for BA. 3.2. Results and discussion The LTs for 6-, 9-, and 12-month-old stimuli were 12.50 s (S.D. = 4.96 s), 13.68 s (S.D. = 3.63 s), and 14.55 s (S.D. = 4.05 s), respectively, for the 6-month-old group, and 14.37 s (S.D. = 4.82 s), 12.57 s (S.D. = 4.18 s), and 13.13 s (S.D. = 4.56 s), respectively, for the 9-month-old group. For both the groups, the ANOVA revealed no significant main effect of the stimuli (F(2, 38) = 2.01, ns, for the 6-month-old group; F(2, 28) = 0.86, ns, for the 9-month-old group). The relative frequencies of BA for 6-, 9-, and 12-month-old infant stimuli were 0.73, 0.61, and 0.47, respectively, for the 6-month-old group, and 0.20, 0.32, and 0.08, respectively, for the 9-month-old group. For both the groups, the ANOVA revealed no significant main effect of the stimuli (F(2, 38) = 0.38, ns, for the 6-month-old group; F(2, 28) = 0.42, ns, for the 9-month-old group). Experiment 3, which used movies transformed with mosaic patterns, suggested that the relative visual activity of a particular stimulus was not effective in making the participants produce different reactions. The results of Experiment 3 suggested that 9-month-old infants showed a preference for 9-month-old infants on the basis of information pertaining to physical appearance, not that pertaining to visual activity. 4. General discussion Both 6- and 9-month-old infants showed sensitivity to the photographs of other same-age infants. Therefore, the sensitivity to similarity to oneself appears to be the plausible explanation for the peer preference of infants. Previous researches have indicated that the visual recognition of self on the basis of featural information emerges during the first 6 months of life (Bahrick, Moss, & Fadil, 1996) and that infants seem to be able to recognize their own visual appearances as familiar stimuli (Bahrick, 1995; Bahrick et al., 1996; Legerstee, Anderson, & Schaffer, 1998). The present findings forge important links among studies of self-recognition and social knowledge. Infants might be able to detect similarity and feel familiarity toward same-age infants on the basis of the self-recognition. In the case of 9-month-old infants, sensitivity to other same-age infants is also observed when the infants are shown movies as stimuli; older infants may be better at processing stimuli that include several kinds of information and at detecting other individuals who are similar to themselves. These findings are supported by another line of previous research. For instance, evidence with regard to categorical object perception indicates that infants begin processing information beyond the independent features of a stimulus during the latter half of the first year of life (Needham & Baillargeon, 1997; Wilcox & Baillargeon, 1998; Younger & Cohen, 1986). During this period, their ability to process information may show a great improvement. Some previous works have assumed that infants can recognize features “like me” (Meltzoff & Brooks, 2001; Meltzoff & Moore, 1995). For instance, in the case of early imitation, infants differentially responded to tongue protrusion with tongue protrusion and not lip protrusion; this revealed that the infants were able to identify the specific organ that needed to be imitated (Meltzoff & Moore, 1977). In this way, human infants seem to detect humans as “like me” individuals, in other words, as conspecifics (Meltzoff, Gopnik, & Repacholi, 1998). The present study could extend these views: besides detecting conspecifics, infants might also detect relative values of similarity between the self and the other. The similarity or difference in physical appearance could be one critical cue to afford such detection. The imitative tendency of infants might also explain why infants were more attracted to other same-age infants. In order to reproduce observed behaviors performed by other individuals, imitators have to adapt the particular (sequence of) behavior to their own bodily characteristics, which differ from those of the model in terms of size, proportion and the range of joint movement. Studies with robots have suggested that it is difficult for a robot agent to adapt the imitated action to the current environment in which it is reproduced (Drescher, 1991; Kuniyoshi, Inaba, & Inoue, 1994). Infants, too, must face such a difficulty, especially when they imitate an adult’s behavior. However, they do imitate various
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kinds of behaviors observed in daily life from the early stages of development (e.g., Field, Woodson, Greenberg, & Cohen, 1982; Heimann, 1989; Meltzoff & Moore, 1977, 1983, 1989; Reissland, 1988); infants seem to overcome the difficulty of paying considerable cognitive costs to reproduce other individual’s behavior. From among the models that infants imitate, the imitation of peers should involve the least cost for the interchange of the behaviors to imitate since in this case, the imitator and imitatee share the most similar physical characteristics. As demonstrated in the previous study, bodily characteristics should serve as an important cue for such peer detection. On the basis of the imitative tendency in infancy (e.g., Meltzoff & Moore, 1989), human infants might detect and prefer individuals who are highly similar to themselves, that is, same-age peers. In the present study, the relative visual activity of the stimuli was not effective in making infants produce different reactions. However, this does not imply that bodily movements are unimportant in the identification of peers. We have not examined the effect of movement patterns in releasing peer preference since the movies transformed with mosaic patterns distorted the qualitative features of the movements to the extent that they were not recognizable as human motion. To examine the possibility that the development of actions plays an important role in infants’ peer preference, movement pattern is one subsequent area that our study should focus on. One suitable way to accomplish this would be by conducting an experiment that uses biological motions focusing on locomotive development such as crawling and walking as the stimuli. Acknowledgements We are grateful to all the models, infants, and caretakers for their participation in this study. 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Meltzoff, A. N., Gopnik, A., & Repacholi, B. M. (1998). Toddlers’ understanding of intentions, desires, and emotions: Explorations of the dark ages. In P. D. Zelazo, J. W. Astington, & D. R. Olson (Eds.), Developing theories of intention: Social understanding and self-control (pp. 17–41). Mahwah, NJ: Lawrence Erlbaum Associates. Meltzoff, A. N., & Moore, M. K. (1977). Imitation of facial and manual gestures by human neonates. Science, 198, 75–78. Meltzoff, A. N., & Moore, M. K. (1983). Newborn infants imitate adult facial gestures. Child Development, 54, 702–709. Meltzoff, A. N., & Moore, M. K. (1989). Imitation in newborn infants: Exploring the range of gestures imitated and the underlying mechanisms. Developmental Psychology, 25, 954–962. Meltzoff, A. N., & Moore, M. K. (1995). A theory of the role of imitation in the emergence of self. In P. Rochat (Ed.), The self in early infancy: Theory and research (pp. 73–93). New York: North-Holland. Nakata, T., & Trehub, S. E. (2004). Infants’ responsiveness to maternal speech and singing. Infant Behavior & Development, 27, 455–464. Needham, A., & Baillargeon, R. (1997). Object segregation in 8-month-old infants. Cognition, 62, 121–149. Negayama, K. (1997). A study of longitudinal change in cuteness of children’s faces (in Japanese). Waseda Studies in Human Sciences, 10, 61–68. Reissland, N. (1988). Neonatal imitation in the first hour of life: Observations in rural Nepal. Developmental Psychology, 24, 464–469. Sanefuji, W., Ohgami, H., & Hashiya, K. (2005). Infants’ preference for infants and adults. In Proceedings of 2005 4th IEEE international conference on development and learning (pp. 93–95). Thelen, E. (1981a). Rhythmical behaviour in infancy: An ethological perspective. Developmental Psychology, 17, 237–257. Thelen, E. (1981b). Kicking, rocking, and waving: Contextual analysis of rhythmical stereotypies in normal human infants. Animal Behavior, 29, 3–11. Wilcox, T., & Baillargeon, R. (1998). Object individuation in young infants: Further evidence with an event-monitoring paradigm. Developmental Science, 1, 127–142. Younger, B. A., & Cohen, L. B. (1986). Developmental change in infants’ perception of correlations among attributes. Child Development, 57, 803–815.
Preference for peers in infancy Wakako Sanefuji a,b,∗ , Hidehiro Ohgami c , Kazuhide Hashiya c a
Graduate School of Human-Environment Studies, Kyushu University, 6-19-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan b Japan Society for the Promotion of Science Research Fellow c Faculty of Human-Environment Studies, Kyushu University, Fukuoka, Japan Received 8 March 2006; received in revised form 17 June 2006; accepted 28 July 2006
Abstract Previous findings suggest that infants prefer other infants from among individuals of various ages. There are two explanations for this: one is that like adults, infants prefer babyish characteristics; the other is that infants prefer individuals who are similar to themselves. We examined whether infants respond differently to same-age infants from among older or younger infants. This might allow the possibility of methodologically separating the two explanations for peer preference in infancy. The results revealed that 9-month-olds showed peer preference when they saw movies of infants. Further experiments showed that 6- and 9-month-olds preferred static images of same-age infants. The lack of preference of 6-month-olds for movies of same-age infants could be related to their ability to process information. Thus, the cue for peer preference may be physical similarity to oneself, rather than the strength of babyish characteristics reflecting a particular developmental stage. © 2006 Elsevier Inc. All rights reserved. Keywords: Infants; Preference; Peers; Similarity; Physical appearance
When infants encounter other individuals, they tend to look at their peers (age mates) eagerly. Infants looked longer at photos, drawings, and movies of infants than at those of older children (Lewis & Brooks, 1975; McCall & Kennedy, 1980; Sanefuji, Ohgami, & Hashiya, 2005). In another study, when a child approached an infant, the infant smiled; however, the opposite occurred when a midget or an adult approached the infant (Brooks & Lewis, 1976). Infants expressed more positive affect, for instance, they smiled, vocalized, and gestured (e.g., banged their arms) more to pictures and movies of infants than to those of older children or adults (Lewis & Brooks, 1975; Sanefuji et al., 2005). All these findings suggest that infants respond differently to individuals of various ages and that they prefer other infants to older children. However, the basis of such peer preference is still under discussion. One explanation for this tendency is the preference for babyish characteristics. Adults tend to exhibit maternal and protective behaviors toward babies. Such behavior may be released by the baby schemas, which refer to infantile characteristics such as the eyes being set at and below the midline of the face, a relatively large head, and clumsy movements (Eibl-Eibesfeldt, 1975; Lorenz (1943) as cited in Shaw & Bransford, 1977). Further, the combination of these infantile characteristics determines the strength of the baby schema. It is suggested that baby schemas are most remarkable just before 1 year of age; studies on adults’ perception of cuteness in human photographs have indicated that infants just below 1 year were selected as being the ∗
Corresponding author. Tel.: +81 92 642 4443; fax: +81 92 642 4443. E-mail address: [email protected] (W. Sanefuji).
0163-6383/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.infbeh.2006.07.007
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cutest (e.g., Hildebrandt & Fitzgerald, 1979; Negayama, 1997). The increase in cuteness at around 1 year of age is likely to be adaptive as a releaser for caretaking behavior, which is considered to be most necessary for infants who have just begun walking independently. Kramer, Zebrowitz, Giovanni, and Sherak (1995) reported that infants as well as adults tended to look longer at a monitor showing an 18-year-old, baby-faced girl than that showing a mature-faced girl of the same age, who was as attractive. They interpreted this result as evidence of the emergence of the preference for baby schemas during infancy. This interpretation should lead to the prediction that just as in the case of adults, from among infants of various ages, infants prefer those that have the strongest babyish characteristics, that is, infants who are around 1 of age. Another interpretation is infants’ preference for others who are similar to themselves. Infants showed a looking preference for photographs and films of other same-sex infants (Aitken (1977) as cited in Bower, 1989; Lewis & Brooks, 1974). On presenting of patch-light displays of a boy and girl infant (biological motion) on film, it was found that the first-fixation measure indicated infants’ significant preference for other same-sex infants (Kujawski & Bower, 1993). Previous studies interpreted their findings as evidence of the ability to identify those infants who share the same gender category (in the sense that they are similar to the infants themselves), not only from feature cues but also from bodily movements. A similar argument has been put forth in imitation studies; infants were capable of representing different types of sensory information on the basis of their ability to match their own behaviors with gestures they observed others performing (Meltzoff & Moore, 1977, 1983, 1995). It is also possible that infants prefer similar-age infants, and not just same-sex infants, from among infants of various ages. Although the two explanations for peer preference are not mutually exclusive, clear evidence regarding which of the two interpretations is more plausible has not been obtained. We attempted to approach this question by investigating infants’ responses to infants of various ages, and not to other individuals of various ages. Thus, the present study examined infants’ responses to the images of infants of various ages displayed on a monitor. Infants with a preference for baby schemas (the baby schemas being judged by adults), irrespective of their ages, are predicted to prefer infants of a particular age with strong babyish characteristics, such as an infant who is approximately 1 year of age (e.g., Hildebrandt & Fitzgerald, 1979; Negayama, 1997). On the other hand, infants with a preference for those similar to themselves are predicted to prefer same-age infants. 1. Experiment 1 In order to investigate which of the two explanations for infants’ preference for other infants is more plausible, Experiment 1 examined infants’ responses to the images of infants of various ages displayed on a monitor. 1.1. Method 1.1.1. Participants Forty infants participated in this experiment: twenty 6-month-olds (12 boys, 8 girls; M = 6.10 months, S.D. = 0.84 months) and twenty 9-month-olds (10 boys, 10 girls; M = 9.16 months, S.D. = 0.78 months). None of the participants had a twin and none of them attended nursery school. Thus, the visual experience of seeing other infants in their daily lives was controlled. The data pertaining to four additional infants were eliminated from the analysis due to the infants’ fussiness. Informed consent was obtained from the caretakers of all the participants. 1.1.2. Stimuli Twelve infants (four 6-, 9-, and 12-month-olds; two boys and two girls for each age group) participated in this experiment as models for the stimuli. All the models were unfamiliar to the participants. Further, all the models were requested to wear identically designed T-shirts and pants; they were provided with three objects (a ball and two toys) to handle (one object was given to the infants at a time). To separate the possibility that a specific way of manipulating an object might influence the infants’ preferential responses, three different objects were provided to the models as stimuli. They handled one object per scene freely against the same background, which implies that 12 types of movies (4 models × 3 objects) were prepared for each age group. These recorded movies were edited to a 20-s movie clip, which had no sound stimuli. Informed consent was obtained from the caretakers of all the models.
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1.1.3. Apparatus The experiment was conducted in a compartment (approximately 155 cm × 110 cm × 155 cm) located in a quiet room, the walls and ceiling of which were covered with a beige cloth, thereby creating a homogeneous background in order to prevent any visual distraction. The monitor (SHARP, LC-20V20, 30 cm × 40 cm), on which the stimuli were displayed, was set on the floor in the compartment. Behind the monitor, a digital video camera was placed to record the responses of the participants during the test session. 1.1.4. Procedure The caretakers sat on the floor at a distance of one meter from the monitor and placed the infants on their lap, using a cuddle seat to set the infants’ position. During the entire test session, the caretaker was requested not to do anything and to move as little as possible. The infants were shown the stimuli of the three different ages one at a time. The trials did not end as soon as the infants looked away; each stimulus was demonstrated for 20 s. The sex of the infant in a stimuli set was the same for each participant; however, the presentation of the stimuli sets depicting infants of either sex was counterbalanced across the participants. The order of the stimuli presentation was also counterbalanced across the participants. 1.1.5. Measurements Using the video camera recordings, we measured the infants’ looking time (LT) and the frequency of banging arms while they looked at the stimuli (BA). LT was defined as the total fixation time for a particular stimulus. Fixation time was measured by an infant’s head orientation. BA was defined as the rhythmical behavior of banging arms largely in the vertical direction. Several kinds of rhythmical behaviors including banging are commonly observed in human infants. Banging stands out for the frequency with which it is observed in children and infants appear to
Fig. 1. The LT in the experiment that used movies as stimuli. Each bar indicates the S.E.s. The upper panel shows the results obtained from the 6-month-old group and the lower panel shows the results obtained from the 9-month-old group.
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be pleasurable in performing this action (Thelen, 1981a). Moreover, a wide variety of relatively non-specific stimuli and internal states appear to facilitate its release (Thelen, 1981b). On the basis of ethological reports (Thelen, 1981a, 1981b), we focused on arm banging behavior, the occurrence of which is largely equal across participants of various ages. Recent studies have begun to use this behavior as a means for measurement in infancy (e.g., Nakata & Trehub, 2004; Sanefuji et al., 2005). In the present study, arm banging served as an indicator of the release of infants’ positive emotion. The relative frequency of BA after controlling the infants’ LT was used in the analysis. As a reliability check, two observers who were blind to the experimental design independently coded 25% of all the data in each experiment. The coding agreements between the two observers were 96.8% for LT and 97.7% for BA. 1.2. Results and discussion The LTs for the 6-, 9-, and 12-month-old infant stimuli were 17.73 s (S.D. = 2.57 s), 17.85 s (S.D. = 2.83 s), and 16.25 s (S.D. = 4.18 s), respectively, for the 6-month-old group, and 17.00 s (S.D. = 2.51 s), 18.35 s (S.D. = 2.13 s), and 16.33 s (S.D. = 2.67 s), respectively, for the 9-month-old group (Fig. 1). An analysis of variance (ANOVA) revealed no significant main effect of the stimuli for the 6-month-old group (F(2, 38) = 2.26, ns). On the other hand, a main effect of the stimuli was observed for the 9-month-old group (F(2, 38) = 3.69, p = 0.034). Nine-month-old infants looked longer at 9-month-old infant stimuli than at 12-month-old infant stimuli (Ryan’s method; t = 2.68, p = 0.01). The relative frequencies of BA while looking at the 6-, 9-, and 12-month-old infant stimuli were 0.90, 0.24, and 0.67, respectively, for the 6-month-old group, and 0.35, 0.82, and 0.18, respectively, for the 9-month-old group (Fig. 2). The ANOVA showed no significant main effect of the stimuli for the 6-month-old group (F(2, 38) = 2.46, ns). On the other hand, for the 9-month-old group, a main effect of the stimuli was observed (F(2, 38) = 3.33, p = 0.046). Nine-month-old
Fig. 2. The relative frequency of BA in the experiment that used movies as stimuli. Each bar indicates the S.E.s. The upper panel shows the results obtained from the 6-month-old group and the lower panel shows the results obtained from the 9-month-old group.
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infants banged their arms more frequently while looking at 9-month-old infant stimuli than at 12-month-old infant stimuli (Ryan’s method; t = 2.45, p = 0.019). On considering the results collectively, it can be said that 9-month-old infants looked longer at 9-month-old infants than at 12-month-old infants. Nine-month-old infants also manifested more behavioral responses while looking at 9month-old infants than at 12-month-old infants who are suggested to have strong babyish characteristics. These results indicate that 9-month-old infants might prefer others who are similar to themselves. On the other hand, 6-month-old infants did not show different response with regard to LT or behavioral responses. There are two possibilities to explain the lack of preferences in 6-month-old infants; one is that a preference for others who are similar to oneself might emerge during the latter period of the first year of life. The other possibility is that 6-month-old infants are unable to deal appropriately with movies that include several types of information such as that pertaining to physical appearance and visual activity. If the first possibility holds true, 6-month-old infants would not show their preferences when any types of information is presented; on the other hand, if the second possibility holds true, 6-month-old infants would show their preferences when other types of information presented. Subsequent experiments were conducted to explore the cause of the lack of preference in 6-month-old infants and the key factor responsible for the preference in 9-month-old infants. 2. Experiment 2 Experiment 2 was conducted to examine the possibility that the physical appearance of the stimuli worked as a cue for infants’ peer preference. 2.1. Method 2.1.1. Participants Thirty-five infants participated in this experiment; nineteen 6-month-olds (10 boys, 9 girls; M = 6.60 months, S.D. = 0.74 months) and sixteen 9-month-olds (8 boys, 8 girls; M = 9.20 months, S.D. = 0.77 months). As in the case of Experiment 1, the daily experience of seeing other infants was controlled among the participants. The data pertaining to seven additional infants were eliminated from the analysis due to the infants’ fussiness. Informed consent was obtained from the caretakers of all the participants. 2.1.2. Stimuli The stimuli used in Experiment 1 were transformed into colored pictures. Due to this, the model’s physical appearance in the original stimulus could be viewed, whereas the relative activity for each original stimulus became impossible to detect. Although more than one picture was prepared from the movies of all the 6-, 9-, and 12-month-old infant stimuli, only one picture of each infant was used. Every picture was displayed for 15 s to sustain the participants’ attention until the end of the presentation. 2.1.3. Apparatus The apparatus used in Experiment 2 was the same as that used in Experiment 1. 2.1.4. Procedure The procedure followed in Experiment 2 was the same as that followed in Experiment 1. 2.1.5. Measurements The measurements used in Experiment 2 were the same as those used in Experiment 1. The coding agreements were 98.1% for LT and 97.2% for BA. 2.2. Results and discussion The LTs for 6-, 9-, and 12-month-old infant stimuli were 9.66 s (S.D. = 3.55 s), 9.82 s (S.D. = 2.91 s), and 9.42 s (S.D. = 3.69 s), respectively, for the 6-month-old infant group, and 10.25 s (S.D. = 3.25 s), 11.91 s (S.D. = 2.49 s), and 9.53 s (S.D. = 3.49 s), respectively, for the 9-month-old infant group (Fig. 3). The ANOVA revealed no
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Fig. 3. The LT in the experiment that used colored pictures as stimuli. Each bar indicates the S.E.s. The upper panel shows the results obtained from the 6-month-old group and the lower panel shows the results obtained from the 9-month-old group.
significant main effect of the stimuli for the 6-month-old group (F(2, 36) = 0.22, ns). On the other hand, a main effect of the stimuli was observed for the 9-month-old group (F(2, 30) = 4.17, p = 0.025). Nine-month-old infants looked longer at 9-month-old infant stimuli than at 6-month-old infant stimuli (Ryan’s method; t = 2.85, p = 0.007). The relative frequencies of BA for 6-, 9-, and 12-month-old infant stimuli were 0.84, 0.25, and 0.17, respectively, for the 6-month-old group, and 0.07, 0.47, and 0.03, respectively, for the 9-month-old group (Fig. 4). The ANOVA was conducted with the stimuli as the independent variable in each group. For the 6-month-old group, a main effect of the stimuli was observed (F(2, 36) = 5.16, p = 0.01). Six-month-old infants banged their arms more frequently while looking at 6-month-old infant stimuli than at 9- or 12-month-old infant stimuli (Ryan’s method; t = 2.44, p = 0.019, and t = 3.03, p = 0.004, respectively). For the 9-month-old group, the stimuli had a main effect (F(2, 30) = 4.01, p = 0.028). Nine-month-old infants banged their arms more frequently while looking at 9-month-old infant stimuli than while looking at 6- or 12-month-old infant stimuli (Ryan’s method; t = 2.37, p = 0.024, and t = 2.53, p = 0.017, respectively). These results suggested that when presented with only physical displays of infant stimuli, both 6- and 9-month-old infants exhibited a preference for looking at same-age infants. This suggests that physical appearance is one of the key factors responsible for 9-month-old infants’ preference for the movies of the same-age infants. However, with regard to the responses of the 6-month-old infants, there appeared to be an inconsistency; 6-month-old infants did not look significantly longer at any stimuli; however, they manifested more behavioral responses to the pictures of same-age infants. Previous studies have suggested that the duration of LT can cover a multitude of looking patterns and this may prevent consideration of important aspects of fixation (Kujawski & Bower, 1993). It is possible that BA can measure some important aspects of fixation that LT is incapable of measuring completely, especially in preverbal period. Additional behavioral responses in 6-month-old infants suggested that they were able to deal appropriately with fewer types of information such as that pertaining to physical appearances and that they show a preference for
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Fig. 4. The relative frequency of BA in the experiment that used colored pictures as stimuli. Each bar indicates the S.E.s. The upper panel shows the results obtained from the 6-month-old group and the lower panel shows the results obtained from the 9-month-old group.
same-age infants. Thus, physical appearances must be important in detecting similarity when 6- and 9-month-old infants compare themselves with other infants. 3. Experiment 3 Experiment 3 was conducted to examine the possibility that the visual activity of the particular stimulus worked as a cue for infants’ peer preference. 3.1. Method 3.1.1. Participants Thirty-five infants participated in this experiment; twenty 6-month-olds (11 boys, 9 girls; M = 6.52 months, S.D. = 0.80 months) and fifteen 9-month-olds (7 boys, 8 girls; M = 9.25 months, S.D. = 0.81 months). As in the case of Experiment 1, the daily experience of seeing other infants was controlled among the participants. The data pertaining to eight additional infants were eliminated from the analysis due to the infants’ fussiness. Informed consent was obtained from the caretakers of all the participants. 3.1.2. Stimuli The stimuli used in Experiment 1 were transformed into movies with mosaic patterns by using a digital AV mixer (Panasonic, WJ-AVE55). Due to this, the relative activity for each original stimulus was represented as dynamic patterns of blinking squires that changed with regard to color and brightness, whereas the physical appearance of the original stimulus was distorted and became impossible to detect. The participants were shown these movies, which were without any sound, for 20 s. 3.1.3. Apparatus The apparatus used in Experiment 3 was the same as that used in Experiment 1.
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3.1.4. Procedure The procedure followed in Experiment 3 was the same as that followed in Experiment 1. 3.1.5. Measurements The measurements used in Experiment 3 were the same as those used in Experiment 1. The coding agreements were 95.3% for LT and 95.6% for BA. 3.2. Results and discussion The LTs for 6-, 9-, and 12-month-old stimuli were 12.50 s (S.D. = 4.96 s), 13.68 s (S.D. = 3.63 s), and 14.55 s (S.D. = 4.05 s), respectively, for the 6-month-old group, and 14.37 s (S.D. = 4.82 s), 12.57 s (S.D. = 4.18 s), and 13.13 s (S.D. = 4.56 s), respectively, for the 9-month-old group. For both the groups, the ANOVA revealed no significant main effect of the stimuli (F(2, 38) = 2.01, ns, for the 6-month-old group; F(2, 28) = 0.86, ns, for the 9-month-old group). The relative frequencies of BA for 6-, 9-, and 12-month-old infant stimuli were 0.73, 0.61, and 0.47, respectively, for the 6-month-old group, and 0.20, 0.32, and 0.08, respectively, for the 9-month-old group. For both the groups, the ANOVA revealed no significant main effect of the stimuli (F(2, 38) = 0.38, ns, for the 6-month-old group; F(2, 28) = 0.42, ns, for the 9-month-old group). Experiment 3, which used movies transformed with mosaic patterns, suggested that the relative visual activity of a particular stimulus was not effective in making the participants produce different reactions. The results of Experiment 3 suggested that 9-month-old infants showed a preference for 9-month-old infants on the basis of information pertaining to physical appearance, not that pertaining to visual activity. 4. General discussion Both 6- and 9-month-old infants showed sensitivity to the photographs of other same-age infants. Therefore, the sensitivity to similarity to oneself appears to be the plausible explanation for the peer preference of infants. Previous researches have indicated that the visual recognition of self on the basis of featural information emerges during the first 6 months of life (Bahrick, Moss, & Fadil, 1996) and that infants seem to be able to recognize their own visual appearances as familiar stimuli (Bahrick, 1995; Bahrick et al., 1996; Legerstee, Anderson, & Schaffer, 1998). The present findings forge important links among studies of self-recognition and social knowledge. Infants might be able to detect similarity and feel familiarity toward same-age infants on the basis of the self-recognition. In the case of 9-month-old infants, sensitivity to other same-age infants is also observed when the infants are shown movies as stimuli; older infants may be better at processing stimuli that include several kinds of information and at detecting other individuals who are similar to themselves. These findings are supported by another line of previous research. For instance, evidence with regard to categorical object perception indicates that infants begin processing information beyond the independent features of a stimulus during the latter half of the first year of life (Needham & Baillargeon, 1997; Wilcox & Baillargeon, 1998; Younger & Cohen, 1986). During this period, their ability to process information may show a great improvement. Some previous works have assumed that infants can recognize features “like me” (Meltzoff & Brooks, 2001; Meltzoff & Moore, 1995). For instance, in the case of early imitation, infants differentially responded to tongue protrusion with tongue protrusion and not lip protrusion; this revealed that the infants were able to identify the specific organ that needed to be imitated (Meltzoff & Moore, 1977). In this way, human infants seem to detect humans as “like me” individuals, in other words, as conspecifics (Meltzoff, Gopnik, & Repacholi, 1998). The present study could extend these views: besides detecting conspecifics, infants might also detect relative values of similarity between the self and the other. The similarity or difference in physical appearance could be one critical cue to afford such detection. The imitative tendency of infants might also explain why infants were more attracted to other same-age infants. In order to reproduce observed behaviors performed by other individuals, imitators have to adapt the particular (sequence of) behavior to their own bodily characteristics, which differ from those of the model in terms of size, proportion and the range of joint movement. Studies with robots have suggested that it is difficult for a robot agent to adapt the imitated action to the current environment in which it is reproduced (Drescher, 1991; Kuniyoshi, Inaba, & Inoue, 1994). Infants, too, must face such a difficulty, especially when they imitate an adult’s behavior. However, they do imitate various
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kinds of behaviors observed in daily life from the early stages of development (e.g., Field, Woodson, Greenberg, & Cohen, 1982; Heimann, 1989; Meltzoff & Moore, 1977, 1983, 1989; Reissland, 1988); infants seem to overcome the difficulty of paying considerable cognitive costs to reproduce other individual’s behavior. From among the models that infants imitate, the imitation of peers should involve the least cost for the interchange of the behaviors to imitate since in this case, the imitator and imitatee share the most similar physical characteristics. As demonstrated in the previous study, bodily characteristics should serve as an important cue for such peer detection. On the basis of the imitative tendency in infancy (e.g., Meltzoff & Moore, 1989), human infants might detect and prefer individuals who are highly similar to themselves, that is, same-age peers. In the present study, the relative visual activity of the stimuli was not effective in making infants produce different reactions. However, this does not imply that bodily movements are unimportant in the identification of peers. We have not examined the effect of movement patterns in releasing peer preference since the movies transformed with mosaic patterns distorted the qualitative features of the movements to the extent that they were not recognizable as human motion. To examine the possibility that the development of actions plays an important role in infants’ peer preference, movement pattern is one subsequent area that our study should focus on. One suitable way to accomplish this would be by conducting an experiment that uses biological motions focusing on locomotive development such as crawling and walking as the stimuli. Acknowledgements We are grateful to all the models, infants, and caretakers for their participation in this study. 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