Doctor of Philosophy in Psychology (PhD) 
Language development in young children
A longstanding question in the literature on language development concerns the nature of infants’ first object word representations: Do these terms have scope over individual objects or entire object categories? Answering this question is important for accounts of the origin of lexical knowledge. Through seven experiments, this dissertation explored the nature of infants’ early object word comprehension. Experiments 1 to 4 assessed six-month-olds’ comprehension of the names for their caregivers. The findings support the claim that these words have scope over individual objects and are consistent with the possibility that infants can represent names for individual objects from the outset of word learning. Experiments 5 and 6 explored whether six- and nine-month-olds comprehend both labels with scope over individual objects (the name for their mother, e.g., "Mommy") and labels with scope over object categories (the word "hand" at six months; the word "ball" at nine months). The results were consistent with these predictions. These discoveries suggest that infants can represent both words for individual objects and words for object categories from the beginning of lexical development. Experiment 7 went a step further than the previous experiments by exploring whether 12- to 15-month-olds comprehend two object labels with differing scope for the same object (i.e., their own pet dog or cat): both a name for the individual object (e.g., "Fido") and a name for the object category (e.g., "dog"). The results indicate that by about their first birthday, infants have the cognitive flexibility to learn multiple words differing in extension (individual object, object category) for the same object. Together, the findings suggest that the capacity to learn both words with individual object scope and words with object category scope is present from the outset of lexical development. Additionally, the demonstration that by one year, infants can learn two words - one with individual scope and one with categorical scope – for the same object, indicates previously undocumented flexibility in their capacity to represent the same physical object both as an individual object and as an instance of an object category. The findings significantly enrich our understanding of the origins of lexical development.
Artifacts are ubiquitous in our lives, and we routinely face the task of tracking them through spatiotemporal and qualitative change. How people reason about the identity of individual artifacts through such changes is not well understood. This dissertation reports nine studies that sought to illuminate human reasoning about individual artifact persistence. Studies 1 to 4 examined whether people's attributions of individual persistence to artifacts depend on the maintenance of the objects' kind. Neither children nor adults systematically judged artifacts to be the same individuals following kind-altering transformations. In contrast, 7-year-olds and adults, but not 5-year-olds, judged animals to be the same individuals following such changes. The findings reveal increasing domain specificity in the importance of maintained kind membership for attributions of individual persistence to artifacts. Studies 5 to 8 explored whether the linguistic expression used to label an artifact affects people's judgments of persistence. Participants learned about scenarios involving a complete part-by-part transformation of an artifact, followed by the reassembly of the original parts to create a second artifact. When the pre-transformation artifact was labeled with a proper name, 5- to 7-year-olds and adults extended the expression to only one post-transformation object, indicating a belief that names pick out artifacts as unique individuals. When the same artifact was labeled with a description, however, people extended the expression to as many objects as matched the description, suggesting a belief that descriptions pick out artifacts in a different manner – namely, as instances of a kind with particular properties. Study 9 assessed whether an artifact's history (being owned by a celebrity or a non-celebrity) influences adults' reasoning about its persistence. Participants read scenarios involving the type of transformation presented in Studies 5 to 8. We found that an artifact's history increased adults' ascriptions of its persisting worth, but it did not influence judgments of its persisting identity. By explicating how people reason about individual artifact persistence, these studies advance our understanding of several broad issues about human cognition, including the nature of our concepts, the influence of language on cognition, and the effects of social factors (i.e., an artifact's history) on conceptual representation.
How do infants initially determine whether a novel object word labels a specific individual (e.g. Madonna) or an instance of a category (e.g., a person)? The research in this dissertation tested the hypothesis that infants assume words for objects from some categories (e.g., people) label individuals (are proper names) but words for objects from other categories (e.g., artifacts) label instances of the category (are count nouns). This assumption could help infants to identify proper names and count nouns in their language, and thereby facilitate the learning of the linguistic proper name/count noun distinction.In a preferential looking task, 16- and 17-month-olds heard a novel word for a target person (a face) or artifact, and their willingness to generalize the word to a non-target object was assessed. In Experiment 1, infants restricted the word to the target object when it was paired with a non-target object from a different category, providing evidence that infants can learn a novel word for the target object in this task. In Experiment 2, infants restricted the word to the target object when both the target and non-target objects were people, but not when they were artifacts from the same category. This finding is consistent with the hypothesis that infants interpret words for people as proper names and words for artifacts as count nouns. In Experiment 3, infants were asked to find the referent of a second novel label in a task identical to Experiment 2. Here, infants restricted their looking to the non-target object when the objects were people, but not when they were artifacts. In Experiment 4, infants did not restrict the novel label to a person (a face) when it was inverted. This result provides evidence that infants’ tendency in Experiment 2 to restrict a label to a particular person was not simply due to the greater perceptual complexity of faces. Together, the findings reveal that infants interpret words for people and words for artifacts differently, raising the possibility that object category distinctions help infants to identify proper names and count nouns in their language and to learn how they are expressed linguistically.
Children’s toys and books provide a rich arena for investigating conceptual flexibility, because they often can be understood to possess an individual identity at multiple levels of abstraction. For example, many toys (e.g., a stuffed Winnie-the-Pooh doll) can be construed either as characters from a fictional world, as physical objects in the real world, or as members of a kind. Similarly, books (e.g., a copy of The House at Pooh Corner) can be construed as instantiations of an abstract intellectual object, as individual physical objects, or as members of a kind. In 4 experiments, 155 4- and 5-year-olds participated in a property extension task, the results of which provide evidence of a rich understanding of multiply instantiated individuals. In Experiment 1, children understood that two representations of a fictional character share certain properties in virtue of their shared character identity, and this sharing does not stem simply from having the same name. In Experiment 2, children demonstrated sensitivity to property origins in making inferences about multiple representations of a fictional character, extending properties from one representation of a character to another when the property was acquired by the character but not when it was acquired by the representation. In Experiment 3, children displayed the same conceptual flexibility and sensitivity to property origins when reasoning about multiple copies of an abstract intellectual object. In Experiment 4, children distinguished kind-based inductive inference from character-based inference, extending properties from one representation of a character to a representation of another character of the same kind when properties were inborn but extending properties only to another representation of the same character when they were acquired by the character. In sum, the present findings revealed previously undocumented conceptual abilities in childhood. First, children use individual identity as well as kind identity as a basis for inferring shared properties. Second, children are sensitive to property origins, distinguishing properties that stem from an object’s identity as an instantiation of an abstract individual from those that stem from its discrete physical object identity and those that stem from its identity as an instance of a kind.
An inherent difficulty for infants learning their first language is that when a caregiver presents a name for an object there are many possible referents of the label. For example, the new word could refer to an individual (proper name), a category (count noun), or a property (adjective). Here I explored how infants might identify a novel word’s lexical category and limit its possible meaning. Research has revealed that infants appear to possess an early expectation that a consistent word applied to multiple objects labels a category, in the manner of count nouns (see Waxman & Gelman, 2010). At the same time, studies suggest that hearing distinct labels for a set of objects can serve to highlight contrasts among the objects (Xu, 1999). Labels can contrast objects from one another in a number of different ways (e.g., contrastive count nouns, adjectives, proper names). Yet some types of contrasts may be more salient for some kinds of objects than for others. Although distinct labels might pick out subordinate categories (i.e., count nouns) or distinguishing properties (i.e., adjectives) for objects of any kind, only in the case of people are distinct labels likely to pick out individuals (i.e., proper names) (see Hall, 2009; Macnamara, 1982). We propose that hearing distinct words highlights contrasts between people more so than contrasts between artifacts or other animals. Fourteen-month-old infants viewed images of emus paired with images of female faces. Infants who heard the same label for every pair behaved like infants who heard no labels, and did not preferentially select a new face or new emu for the referent of the same word. In contrast, when infants heard a different label for each face-emu pair, they selected a new face as the referent of a novel word. These findings are consistent with the possibility that infants inferred that distinct labels function to pick out contrasts between people as opposed to contrasts between nonhuman animals. We will discuss how these results bear on the issue of how infants learn the way in which words from different lexical classes are marked in their language.