Whether finding a classroom, meeting a new teacher, learning to add, or reading a story, children understand the world through their concepts. Concepts are crucial for understanding the world because they represent current experiences as belonging to a category of similar experiences. By having a concept chair, a student who sees a new chairat his desk need not re-discover whether it is alive, whether he should write with it or sit on it, or what the teacher means by “take your seat” when pointing at the new chair. In this way, the concept of chairs—rather the sight of a particular chair— allows the mapping of an open-ended number of appropriate reactions and inferences about chairs onto an open-ended number of particular chairs.
Central concept acquisition is key to cognitive development, and children learn actually many concepts. The most conservative estimate of children's concepts would be the number of words for which they know the meaning— roughly 40,000 by age 10 and 60,000 by age 19, which over the first two decades of life is roughly one new concept every 90 waking minutes (Anglin, 1993; Bloom, 2000; Miller, 1996). In truth, this figure radically underestimates the rate of concept acquisition because words often denote separate concepts (e.g., the mole found in the ground is not the same mole found working for an intelligence agency), and many concepts are expressed in word-combinations (bunk bed, riverbed, flowerbed), in predicates (e.g., old enough to run for president), and in morphemes (e.g., -ed in the English past tense or -s in the English plural). Further, judging from nonhuman animals', infants', and languageless adults' performance on learning and memory tasks, many non-verbal concept-like representations (subject, agency, action, cause, consequence, more/less, near/far) appear to exist in absence of language (Fodor, 1975; Furth, 1966; Hauser, 2000; Spelke, 1994). Clearly, even the most ambitious adult could not teach children even a small portion of the concepts that they actually acquire by age 10.
As might be expected, then, most concepts are learned not through direct instruction but through children's experiences. As they encounter the who, what, where, when, why, how, and how many of everyday events, children accumulate information about categories of people, objects, locations, time, causes, functions, and numbers. Although development of concepts pertaining to each of these categories deserves special treatment, certain common trends have also emerged. Three general trends in conceptual development are described below.
First, children increasingly weight defining features of nominal kinds. Although it is difficult for most adults (and even dictionaries) to list necessary and sufficient features for natural kind concepts such as oak, octopus, and onyx, adults find it easier to define nominal kind concepts, such as odd number, uncle, and island. Unlike natural kind concepts, nominal kind concepts follow simple rules, for example, “A number is odd if and only if it is not evenly divisible by two” (Schwartz, 1977). Development of nominal kind concepts was particularly interesting to developmental psychologists because concepts presented such a straightforward test of Vygotsky's idea that “grouping of objects on the basis of maximum similarity is superseded by grouping on the basis of a single attribute” (1986, pp. 136–137) and of Piaget's idea that children's initial categories group things on the basis of accidental rather than essential features (Inhelder & Piaget, 1964).
To test this idea, Keil (1989) presented children with descriptions of nominal kinds that lacked defining features of the category but possessed characteristic features or that lacked the characteristic features of the category but possessed the defining features. For example, when testing children's concept of island, Keil found that kindergarteners typically said that a “place that sticks out of land like a finger” with coconut trees and palm trees was an island— despite lacking defining features of an island, whereas a “place that is surrounded with water on all sides” and covered in snow was not an island—despite possessing defining features of an island. In contrast, second graders typically recognized that palm trees and snow were inessential characteristics of islands and based their judgment of island-hood strictly on the defining features. Moreover, Keil found this age-difference across most of the nominal kind concepts tested, with young children variously averring that “pancakes can't be lunch” (even if eaten at noon) and that 2-year-olds cannot be uncles (even if brothers of somebody's mother).
Although the “characteristic-to-defining features shift” is an important trend in development of nominal kind concepts, subsequent research showed that the shift does not accompany qualitative changes in conceptual representation that Vygotsky and Piaget theorized. First, although adults' knowledge of definitions might allow them to categorize atypical category members (e.g., whales as mammals; Armstrong et al., 1983), even adults can be swayed by characteristic features for conjunctive concepts. For example, definitions imply propositions like “Rover is a dog-and-pet [the conjunctive concept] because Rover has the defining features of dogs and pets.” Yet, when making judgments about conjunctives, adults typically weight characteristic over defining features (e.g., judging chess as a game-and-sport but not a sport; Hampton, 1997). Also, even 5-year-olds make surprisingly philosopher-worthy judgments regarding such moral concepts as lying and stealing (e.g., by recognizing that a boy unpopular for his goodness is still lying when he pretends—uncharacteristically—to be bad; Keil, 1989). These observations are not consistent with Vygotsky's and Piaget's broader claims about conceptual development.
Second, children become increasingly sensitive to the statistical structure of the environment. By their nature, concepts store information about correlations among features, and for good reason: Features well correlated with a category are more reliable cues to category-membership (e.g., wagging tail, furry, barking are correlated with dogs) than features that are not well correlated with the category (e.g., brown-colored is not very well correlated with being a dog since many non-dogs are brown and many dogs are not brown). Barking, for example, would have higher cue validity than being brown-colored, and sensitivity to this cue validity would allow recognition of category-membership most reliably. This principle applies to natural and nominal kind concepts. For example, being older-than-2-years-old would not be a feature of uncle because many non-uncles are older-than-2-years-old.
Children of all ages (even infants) are sensitive to the statistical distribution of features over natural categories (Quinn & Eimas, 1996; Rosch et al., 1976), and this fact explains many aspects about infants' perceptual categories and older children's concepts. For example, basic-level categories (dog, table, and car) possess features with higher cue validities than both superordinate categories (animal, furniture, and vehicles), which have few features in common, and subordinate categories (collie, coffee-table, and Corvette), which have many features in common with contrasting subordinate categories (Rosch et al., 1976). This is important for conceptual development: Children learn basic-level categories most easily (Horton & Markman, 1980), learn names of basic-level categories earlier than names of superordinate and subordinate categories (Anglin, 1977), are most likely to interpret novel words as basic level categories (Callanan, 1989), and are most likely to generalize novel properties over basic level categories (Gelman & O'Reilly, 1988). Further, category-members differ in the number of features that have high cue validity for that concept, with prototypical category-members having the most such features. For example, robins are more prototypical birds than are ostriches because robins fly and flight has high cue validity for the bird category. Again, this difference in cue validity is important in the development of category recognition, with younger children typically claiming that robins are birds (but ostriches are not). Older children and adults show a similar effect of cue validity: robins are recognized as birds more quickly than ostriches (Rips et al., 1973). Finally, even infants as young as 3 months can rapidly form category prototypes from brief experiences with novel categories (Bomba & Siqueland, 1983), and adults judge even abstract categories to have prototypical members (e.g., judging 7 a better example of “odd number” than 23; Armstrong et al., 1983).
Although infants and young children are initially quite sensitive to the statistical distribution of many features, much of the statistical structure of the world passes beneath their notice and requires intervention by adult experts. This is especially true of features that are not perceived, either because the feature is perceptible in principle but not in actuality (e.g., because it is too subtle to notice, too small, or too far away) or because the feature is abstract and not even perceptible in principle (e.g., the fairness of a rule, the truth of a statement, or the product of two quantities). Toddlers notoriously overlook subtle properties (such as wicks on candles) when categorizing objects, unless an experimenter explains how the property correlates with its function (Tver-sky & Hemenway, 1984; Banigan & Mervis, 1988).
The ability to notice correlations among imperceptible properties is especially important in mathematical and scientific reasoning. For example, although children cannot see the movement of plants (because they move too slowly to be perceived), knowing abstractly that plants move helps older children realize that plants are living things like animals (Opfer & Gelman, 2001; Opfer & Siegler, 2004). One cannot directly perceive torque (the product of the weight of an object and its distance from the fulcrum), yet torque tells us which of two sides of a balance beam will go down, and sensitivity to the correlation between torque and balance increases quite dramatically with age and education (Siegler, 1976). Finally, many features are relative (e.g., having more bristles than eyes), and when learning artificial categories, recognition of cue validity of relative features improves with development (Sloutsky, Kloos, & Fisher, 2007).
Third, children become increasingly sensitive to the causal structure of the environment. Beyond representing information about the statistical structure of the world, concepts also retain information about causal structure. Children do not simply know that birds build nests and can fly and have feathers and are bird-offspring, they also know that birds build nests because they can fly, can fly because they have feathers, and have feathers because they are bird-offspring. Acquiring causal information is important for learning, memory and generalization (Murphy & Medin, 1985).
Children show early evidence of their sensitivity to the causal structure of the environment, allowing them to better learn and remember categories. Provided with a causal theory explaining how the features of fictitious animals were related to their behaviors (e.g., “wugs need armor for fighting” and “gillies need big ears for hiding”), 4-year-olds better remembered the feature/category associations than those who only learned that wugs have armor and gillies have big ears but not why (Krascum & Andrews, 1998). The implication is that children provided with causal information explaining why features co-occur remember categories better than if they have only learned what features co-occur.
Older children become increasingly sensitive to the relative importance of causal similarities. In one study, preschoolers were asked to label, infer novel properties, and project future appearances of a novel animal that varied in two opposite respects: (1) how much it looked like another animal whose name and properties were known, and (2) how much its parents looked like parents of another animal whose name and properties were known. When origins were known, preschoolers generalized to animals with similar origins rather than with similar appearances; when origins were unknown, preschoolers generalized to animals with similar appearance. Results imply that preschoolers actively choose the similarities that best predict accurate generalization (Opfer & Bulloch, 2007). This ability also improves over time. For example, when told that “pizers” have “blickem” in their blood that causes them to have small lungs and purple skin, 9-year-olds are more likely to use causal features, such as “blickem,” to judge category membership than to use an effect, such as purple skin or small lungs (Ahn et al., 2000).
Sensitivity to the causal structure of the world also leads children to hold essentialist beliefs, that is, an idea that categories in the world are based on a true nature that gives an object its identity (Gelman, 2003). These essentialist beliefs can be helpful in leading preschoolers to realize that a pig's insides are more like that of a cow than a piggy bank (Gelman & Wellman, 1991). However, they can also lead to mistaken beliefs about an underlying true nature for social groups and genders (Heyman & Gelman, 2000; Gelman, Collman & Mac-coby, 1986).
At times, young children express funny beliefs that can take them many years to overcome, such as the notion that “pancakes can't be lunch”, that “plants are not alive,” and that pink barrettes can turn a boy into a girl. From these examples, it is tempting to think that children's concepts differ qualitatively from those of adults. Errors such as these can be viewed instead as part of continuous trends in conceptual development, a process that neither begins nor ends at the driveway of their schoolhouse.
See also:Concept Learning
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