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One of the primary functions of natural kind terms (e.g., tiger, gold) is to support inductive inferences. People expect members of such categories to share important, unforeseen properties, such as internal organs and genetic structure. Moreover, inductions can be made without perceptual support: even when an object does not look much like other members of its category, and even when a property is unobservable. The present work addresses how expectations about natural kinds originate. Young children, with their usual reliance on perceptual appearances and only rudimentary scientific knowledge, might not induce new information within natural kind categories. To test this possibility, category membership was pitted against perceptual similarity in an induction task. For example, children had to decide whether a shark is more likely to breathe as a tropical fish does because both are fish, or as a dolphin does because they look alike. By at least age 4, children can use categories to support inductive inferences even when category membership conflicts with appearances. Moreover, these young children have partially separated out properties that support induction within a category (e.g., means of breathing) from those that are in fact determined by perceptual appearances (such as weight). Since

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2 factors were proposed to affect awareness of one's comprehension failure: the inferential processing requirements, and the kind of standards against which comprehension is evaluated. These studies investigated elementary school children's awareness of their own comprehension failure when presented with inconsistent information. Study 1 showed that children were more likely to notice explicit than implicit contradictions. However, even 12-year-olds judged as comprehensible a sizable proportion of essays with seemingly obvious inconsistencies. Yet, the children had good probed recall of the information, the logical capacity to draw the inferences, and were not generally reluctant to question the experimenter. In subsequent studies children were (a) asked to repeat sentences in order to guarantee that the 2 inconsistent propositions were concurrently activated in working memory, and (b) warned about the existence of a problem in order to promote more careful evaluation. Taken together, the results suggest that to notice inconsistencies children have to encode and store the information, draw the relevant inferences, retrieve and maintain the (inferred) propositions in working memory, and compare them. Third through sixth graders do not spontaneously carry out those processes that they are capable of carrying out.

A number of theorists have argued that the productive naming explosion results from advances in abilities that underlie language learning (e.g., the realization that words are symbols, changes in conceptual structure, or the onset of word learning constraints). If any of these accounts are accurate, there should be parallel developments in comprehension. To explore this issue, 4 studies assessed whether pre-and postnaming explosion children differ in their ability to learn a new word after limited exposure. Thirteen-and 18-month-olds heard a new object label just 9 times in a 5-min training session and then their comprehension was assessed in a multiple-choice procedure. Under favorable testing conditions, both 18-and 13-month-olds showed comprehension of the new word, even after a 24-hr delay. These results suggest that well before the productive naming explosion, children can learn a new object label quickly.

This paper views lexical acquisition as a problem of induction: Children musl figure out the meaning of a given term, given the large number of possible meanings any term could have. If children had to consider, evaluate, and rule out an unlimited number of hypotheses about each word in order to figure out its meaning, learning word meanings would be hopeless. Children-must, therefore, be limited i n the kinds of hypotheses they consider as possible word meanings. This paper considers three possible constraints on word meanings: (1) The whole object assumption which leads children to interpret novel terms as labels for objects -not parts, substances, or other properties of objects: (2) The taxonomic assumption which leads children to consider labels as referring to objects of like kind, rather than to objects that are thematically related: and (3) The mutual exclusivity assumption which leads children to expect each object to have only one label. Some of the evidence for these constraints is reviewed. This paper is based on a talk given in the symposium Structural Constraints on Cognitive Development, Psychonomics, 1986 and borrows heavily from Markman (in press). This work was supported in part by NIH Grant HD 20382. I would like to thank Roche1 Gelman, Douglas Medin, Steven Pinker, and Elizabeth Spelke for their thoughtful comments on this manuscript.Correspondence and requests for reprints should be sent to Ellen M. Markman, Psychology

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