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Individuals from five primate species were tested experimentally for their ability to follow the visual gaze of conspecifics to an outside object. Subjects were from captive social groups of chimpanzees, Pan troglodytes, sooty mangabeys, Cercocebus atys torquatus, rhesus macaques, Macaca mulatta, stumptail macaques, M. arctoides, and pigtail macaques, M. nemestrina. Experimental trials consisted of an experimenter inducing one individual to look at food being displayed, and then observing the reaction of another individual (the subject) that was looking at that individual (not the food). Control trials consisted of an experimenter displaying the food in an identical manner when the subject was alone. Individuals from all species reliably followed the gaze of conspecifics, looking to the food about 80% of the time in experimental trials, compared with about 20% of the time in control trials. Results are discussed in terms of both the proximate mechanisms that might be involved and the adaptive functions that might be served by gaze-following. Copyright 1998 The Association for the Study of Animal Behaviour. Copyright 1998 The Association for the Study of Animal Behaviour.

Evidence for primates' understanding of causality is presented and discussed. Understanding causality requires the organism to understand not just that two events are associated with one another in space and time, but also that there is some `mediating force' that binds the two events to one another which may be used to predict or control those events (e.g. a physical force such as gravity or a psychological force such as an intention). In the physical domain, studies of tool use indicate that capuchin monkeys do not have a causal understanding of the functioning of tools in terms of the physical forces involved, but rather they learn to associate aspects of their own behavior with the results it produces. Apes show some possible signs of understanding the causal relations involved in tool use in the sense that they may employ various forms of foresight in approaching novel tasks, perhaps involving an understanding of physical forces-although not to the extent of human children. In the psychological domain, nonhuman primates understand conspecifics as animate beings that generate their own behavior and, thus, they appreciate that to manipulate conspecifics communicative signals, and not physical activities, are required. However, there is very little evidence that nonhuman primates of any species understand others as psychological beings with intentions and other psychological states that mediate their behavioral interactions with the world-as human children begin to do sometime during their second year of life. More research, using a wider range of problem-solving situations, is needed if we are to become more precise in our understanding of how primates understand the causal structure of the world around them.

At around 1 year of age, human infants display a number of new behaviors that seem to indicate a newly emerging understanding of other persons as intentional beings whose attention to outside objects may be shared, followed into, and directed in various ways. These behaviors have mostly been studied separately. In the current study, we investigated the most important of these behaviors together as they emerged in a single group of 24 infants between 9 and 15 months of age. At each of seven monthly visits, we measured joint attentional engagement, gaze and point following, imitation of two different kinds of actions on objects, imperative and declarative gestures, and comprehension and production of language. We also measured several nonsocial-cognitive skills as a point of comparison. We report two studies. The focus of the first study was the initial emergence of infants' social-cognitive skills and how these skills are related to one another developmentally. We found a reliable pattern of emergence: Infants progressed from sharing to following to directing others' attention and behavior. The nonsocial skills did not emerge predictably in this developmental sequence. Furthermore, correlational analyses showed that the ages of emergence of all pairs of the social-cognitive skills or their components were inter-related. The focus of the second study was the social interaction of infants and their mothers, especially with regard to their skills of joint attentional engagement (including mothers' use of language to follow into or direct infants' attention) and how these skills related to infants' early communicative competence. Our measures of communicative competence included not only language production, as in previous studies, but also language comprehension and gesture production. It was found that two measures--the amount of time infants spent in joint engagement with their mothers and the degree to which mothers used language that followed into their infant's focus of attention--predicted infants' earliest skills of gestural and linguistic communication. Results of the two studies are discussed in terms of their implications for theories of social-cognitive development, for theories of language development, and for theories of the process by means of which human children become fully participating members of the cultural activities and processes into which they are born.

Forty-eight young children (2.5 and 3.0 years old) and 9 great apes (6 chimpanzees and 3 orangutans) participated in a hiding-finding game. An adult human experimenter (the Hider) hid a reward in 1 of 3 opaque containers aligned on a wooden plank. Another adult experimenter (the Communicator) attempted to help the subject find the reward by giving 1 of 3 types of communicative sign: (1) Pointing, for which she placed her hand directly above the correct container with index finger oriented down; (2) Marker, for which she placed a small wooden block on top of the correct container; and (3) Replica, for which she held up a perceptually identical duplicate of the correct container. At both ages, children were above chance in this finding game with all 3 types of communicative sign, with Pointing being easiest (because they knew it prior to the experiment), Marker being next easiest, and Replica being most difficult. In contrast, no ape was above chance for any of the communicative signs that it did not know before the experiment (some had been trained in the use of the marker previously, and one knew pointing), nor was group performance above chance for any of the signs, despite the fact that apes experienced three times as many trials as children on each sign. Our explanation of these results is that young children understand the communicative intentions of other persons--although they may have more difficulty comprehending the exact nature of those intentions in some cases--whereas apes treat the behavioral signs of others as predictive cues only (signals). This may be because apes do not perceive and understand the communicative intentions of others, at least not in a human-like way.

Four studies examined English-speaking children's productivity with word order and verb morphology. Two- and 3-year-olds were taught novel transitive verbs with experimentally controlled argument structures. The younger children neither used nor comprehended word order with these verbs; older children comprehended and used word order correctly to mark agents and patients of the novel verbs. Children as young as 2 years 1 month added -ing but not -ed to verb stems; older children were productive with both inflections. These studies demonstrate that the present progressive inflection is used productively before the regular past tense marker and suggest that productivity with word order may be independent of developments in verb morphology. The findings are discussed in terms of M. Tomasello's (1992a) Verb Island hypothesis and M. Rispoli's (1991) notion of the mosaic acquisition of grammatical relations.

A fundamental question of child language acquisition is children's productivity with newly learned forms. The current study addressed this question experimentally with children just beginning to combine words. Ten children between 1;6 and 1;11 were taught four new words, two nouns and two verbs, over multiple sessions. All four words were modelled in minimal syntactic contexts. The experimenter gave children multiple opportunities to produce the words and made attempts to elicit morphological endings (plural for nouns, past tense for verbs). Overall, children combined the novel nouns productively with already known words much more often than they did the novel verbs-by many orders of magnitude. Several children also pluralized a newly learned noun, whereas none of them formed a past tense with a newly learned verb. A follow-up study using a slightly different methodology confirmed the finding of limited syntactic productivity with verbs. Hypotheses accounting for this asymmetry in the early use of nouns and verbs are discussed.

Previous studies have demonstrated that children aged 2;0 can learn new words in a variety of non-ostensive contexts. The current two studies were aimed at seeing if this was also true of children just beginning to learn words at 1;6. In the first study an adult interacted with 48 children. She used a nonce word to announce her intention to find an object ('Let's find the gazzer'), picked up and rejected an object with obvious disappointment, and then gleefully found the target object (using no language). Children learned the new word as well in this condition as in a condition in which the adult found the object immediately. In the second study the adult first played several rounds of a finding game with each of 60 children, in which it was first established that one of several novel objects was always in a very distinctive hiding place (a toy barn). The adult then used a nonce word to announce her intention to find an object ('Let's find the toma') and then proceeded to the barn. In the key condition the barn was mysteriously 'locked'; the child thus never saw the target object after the nonce word was introduced. Children learned the new word as well in this condition as in a condition in which the adult found the object immediately. The results of these two studies suggest that from very early in language acquisition children learn words not through passive, associative processes, but rather through active attempts to understand adult behaviour in a variety of action and discourse contexts.

Fourteen juvenile and adult orangutans and 24 3- and 4-year-old children participated in 4 studies on imitative learning in a problem-solving situation. In all studies a simple to operate apparatus was used, but its internal mechanism was hidden from subjects to prevent individual learning. In the 1st study, orangutans observed a human demonstrator perform 1 of 4 actions on the apparatus and obtain a reward; they subsequently showed no signs of imitative learning. Similar results were obtained in a 2nd study in which orangutan demonstrators were used. Similar results were also obtained in a 3rd study in which a human encouraged imitation from an orangutan that had previously been taught to mimic arbitrary human actions. In a 4th study, human 3- and 4-year-old children learned the task by means of imitation.

We report 3 studies of the referential pointing of 2 orangutans (Pongo pygmaeus). Chantek was raised in an enculturated environment; Puti, raised in a nursery, had a more typical captive life. In Experiment 1, flexibility of pointing behavior was investigated by requiring subjects to point in novel circumstances (for an out-of-sight tool, not food). In Experiment 2, we investigated the orangutans' comprehension of the significance of a human point in helping them to locate food. In Experiment 3, we investigated whether these pointing subjects comprehended that a human recipient must be looking for the point to achieve its attention-directing goal. In all experiments the enculturated orangutan showed better understanding of pointing than the captive orangutan. This finding is consistent with recent studies that have found differences in the cognitive and social-cognitive abilities of apes that have had different types of experience with humans.

In this study we compared the abilities of chimpanzees and human children to imitatively learn novel actions on objects. Of particular interest were possible differences between chimpanzees raised mostly with conspecifics (mother-reared) and chimpanzees raised in a human-like cultural environment (enculturated). Subjects were thus 3 enculturated and 3 mother-reared chimpanzees, along with 8 18-month-old and 8 30-month-old human children. Each subject was tested over a 2-day period with 16 novel objects. The introduction of each object was preceded by a baseline period in which the subject's natural proclivities toward the object were determined. For 12 objects, a human experimenter demonstrated first a simple and then a complex novel action, instructing the subject in each case to "Do what I do" (chimpanzees were prepared for the task behaviorally as well). For the other 4 objects, demonstration of a single action took place on the first day and the subject's opportunity to imitate was delayed until the second day, 48 hours later. Actions that a subject produced in baseline were excluded from further analysis. For each analyzed action, the subject's behavior was scored as to whether it successfully reproduced (1) the end result of the demonstrated action, and (2) the behavioral means used by the demonstrator. Results showed that in immediate imitation the mother-reared chimpanzees were much poorer imitators than the enculturated chimpanzees and the human children, who did not differ from one another. Surprisingly, on the delay trials, the enculturated chimpanzees significantly outperformed the other 3 groups. We conclude from these results that a human-like sociocultural environment is an essential component in the development of human-like social-cognitive and imitative learning skills for chimpanzees, and perhaps for human beings as well.

Common chimpanzees (Pan troglodytes) and 2-year-old human children (Homo sapiens) were presented with a rakelike tool and a desirable but out-of-reach object. One group of subjects observed a human demonstrator use the tool in one way, and another group observed a demonstrator use the tool in another way. Children in both cases did what the model did. Chimpanzee subjects, however, behaved identically in the 2 model conditions. Both groups performed better than subjects who saw no demonstration. This pattern of results suggest that the chimpanzees were paying attention to the general functional relations in the task and to the results obtained by the demonstrator but not to the actual methods of tool use demonstrated. Human children were focused on the demonstrator's actual methods of tool use (her behavior). The different social learning processes used by the 2 species have implications for their different forms of social organization.

Two studies of verb learning are reported. The focus of both studies was on children in their second year of life learning verbs in various pragmatic contexts. Of particular interest was the comparison of ostensive contexts--in which word and referent were simultaneously present in the child's perceptual field--to non-ostensive contexts. In a naturalistic study of 24 children at 1;3 and 1;9, it was found that mothers modelled verbs for their children most often BEFORE the referent action actually occurred. Over 60% of maternal verbs were used to refer to actions that mothers wished children to perform or that they were anticipating their performing (IMPENDING actions). Some verbs were also used to refer to current actions (ONGOING actions) or actions that had just been completed (COMPLETED actions). Children responded with comprehension most often to impending models. Impending and completed models, but not ongoing models, were correlated with children's verb vocabularies at 1;9. The second study was a lexical training study of 48 two-year-olds. Children learned to produce a novel verb best when it was modelled in the impending condition. They learned to comprehend it equally well in the impending and completed conditions. Children showed no signs of superior learning in the ostensive (ongoing) learning context. Results of the two studies are discussed in terms of the different learning processes involved in acquiring nouns and verbs, and, more broadly, in terms of a social-pragmatic view of language acquisition in which the ostensive teaching paradigm is but one of many contexts in which children learn to establish a joint attentional focus with mature language users.

In this study we compared the conversations of mothers and fathers with their children at 1; 3 and 1; 9, with special attention to breakdown-repair sequences. We found that, overall, children and secondary caregiver fathers experienced more communicative breakdowns than did children and primary caregiver mothers. More specifically, fathers requested clarification of their children more often than did mothers, and they most often used a non-specific query (e.g. What?). Mothers used more specific queries (e.g. Put it where?) and were involved in more 'looped' sequences involving multiple requests for clarification. Fathers also failed to acknowledge child utterances more often than did mothers. After a father non-acknowledgement, children tended not to persist and when they did they often received further non-acknowledgements; the dyad did not often return to the child's original topic. After a maternal non-acknowledgement, on the other hand, children persisted and the dyad more often returned to its previous topic. The results are interpreted as support for the Bridge Hypothesis which claims that fathers present children with communicative challenges that help prepare them for communication with less familiar adults.

The peer interactions of 6 infant chimpanzees (Pan troglodytes) ranging in age from 18 to 50 months were observed in a seminatural context. The infants and their mothers lived as members of a captive social group at the Yerkes Regional Primate Research Center. An analysis of contact initiations between infants indicated that the most preferred peer interactant was the youngest and the least preferred was the oldest infant. Infants also initiated more interactions with the offspring of adults that had the closest relationships with both themselves and their mothers. These results indicate that a number of factors may influence the peer affiliations of infant chimpanzees, including the age of the infant and the mother's social relationships.

Initial characterizations of the communicative abilities of preschoolers stressed their egocentric nature. Recently, however, even 2-year-olds have been observed to adjust their speech appropriately in situations in which the listener provides feedback by signaling noncomprehension. The current study had an adult signal noncomprehension to the requests of 2-year-old Stage I and Stage II children. Each child interacted with a familiar (mother) and an unfamiliar adult. The children repeated their requests about one third of the time and revised them about two thirds of the time. Stage I children elaborated their requests significantly more often than Stage II children. The familiarity of the adult listener had no effect on the way Stage II children revised their requests, but the Stage I children's revisions contained novel lexical items more often when they were interacting with the unfamiliar adult. Both of these findings may have resulted from the fact that the more conversationally skilled Stage II children relied on verbal-conversational cues, which were the same for both adult interactants in this situation. The Stage I children may have been less aware of these conversational cues, relying on general social cues such as familiarity of the interactant. The results are discussed in terms of the potential role of different types of adults in the language acquisition process.

A captive chimpanzee group was observed in order to determine the extent to which the social interactions of the infants and juveniles (18-50 months) were affected by their mothers' relationships with other adult group members. It was found that the young chimpanzees initiated more interactions with adults who interacted more with their mothers. A vast majority of those interactions occurred at significant distances from the mother. It is argued that these data imply a social-cognitive ability in young chimpanzees closely related to the human infant's ability to use its mother in 'social referencing'.

This paper reports 2 studies that explore the role of joint attentional processes in the child's acquisition of language. In the first study, 24 children were videotaped at 15 and 21 months of age in naturalistic interaction with their mothers. Episodes of joint attentional focus between mother and child--for example, joint play with an object--were identified. Inside, as opposed to outside, these episodes both mothers and children produced more utterances, mothers used shorter sentences and more comments, and dyads engaged in longer conversations. Inside joint episodes maternal references to objects that were already the child's focus of attention were positively correlated with the child's vocabulary at 21 months, while object references that attempted to redirect the child's attention were negatively correlated. No measures from outside these episodes related to child language. In an experimental study, an adult attempted to teach novel words to 10 17-month-old children. Words referring to objects on which the child's attention was already focused were learned better than words presented in an attempt to redirect the child's attentional focus.