Cognifive Development, 5, 33 l-339 (1990)

Comment on Behrend’s “Constraints and Development”

Katherine Nelson City University of New York Graduate Center

Behrend (1990) correctly perceives that my article (Nelson, 1988) was designed to examine the assumptions of constraints theories in order to clarify the impor- tant issues involved. Unfortunately, on several counts I believe that Behrend’s commentary has muddled rather than clarified. I therefore want to take this op- portunity to address these points further.

First, regarding the empirical record, a number of statements in Behrend’s commentary reflect misunderstandings of my argument, the research basis for it, or both. The point with respect to the characteristics of the first stage of word acquisition (from about IO-13 months to 16-20 months usually) is that these characteristics provide clues to how children begin to make sense of the language they are learning. Briefly, as pointed out in the original article, they are usually slow to acquire words, use them infrequently, comprehend much more than they produce, engage in some seemingly bizarre applications of the words they know (overextension or complexive-take your pick), stop using words previously learned, and exhibit considerable variability (as well as some commonality) in the language forms they acquire. These characteristics are not easily reconciled with the notion of an object class-word constraint, unless the constraint is held to “kick in” later in development. The latter assumption seems ad hoc and mal- adaptive, since, as noted originally, the greatest need for such a constraint would seem to be at the beginning of word learning when the child has the fewest clues (from syntactic structure and preexisting lexical knowledge) as to the meaning of a given word. There is no evidence that I know of that has examined the process of word acquisition during this period that suggests that children begin (indepen- dently of any social cues) by hypothesizing that words are object names.

The picture changes considerably toward the middle and into the second half of the second year when typically (among standard middle-class Western chil- dren) vocabularies expand exponentially, grammatical constructions appear, and

I am indebted to June Hampson for substantive suggestions for inclusion in this commentary. Correspondence and requests for reprints should be sent to Katherine Nelson, Developmental

Rychology Program, City University of NewYork Graduate Center, 33 W. 42nd St., New York, NY 10036.

Manuscript received November 7, 1989; revision accepted January 15, 1990 331

332 Katherine Nelson

children enter into dialogues with their caretakers. Any theory of lexical acquisi- tion should at least attempt to account for these changes.’ Of course I recognize that constraints theorists, like other developmental psychologists, expect changes in the state of the child’s lexicon. The point is that their chosen mechanism for explaining changes-constraints on word meanings-does not change over time and cannot explain qualitative changes. For example, if verbs are “the thing to be doing” at 20 months, as Behrend asserts (in line with Bates, Bretherton, & Snyder, 1988) the object name constraint will hinder rather than help the process.

I have suggested in contrast that the long period of fits and starts toward language is devoted to learning what words do, namely, that they can refer to objects and actions and can be used to request, name, note similarities among things, and so on, as well as regulate actions among people and express affect. Children explore language during this period in different ways, some by produc- ing words and extending them in novel directions, others by listening, and some children have more problems with entering language than others. Importantly, children vary in both the forms they learn and the functions they express in language when they first begin to talk.

In line with most claims of constraints theorists, Behrend denies the vari- ability among children in early vocabulary acquisition that has become so appar- ent to most observers over the past 20 years. My own data (Nelson, 1973) brought this variability to the attention of researchers, but it has since been replicated and substantiated by many different studies (see reviews by Bates et al., 1988; Bretherton, McNew, Snyder, & Bates, 1983; Goldfield & Snow, 1985; McCabe, 1989; Nelson, 1981; Wells, 1986). These studies all agree that a very significant number of American middle-class children do not enter language by learning predominantly nouns referring to object classes. The most recent addi- tion to this literature suggests even more caution in attributing an object name constraint to all first word learners. Hampson (1989a, 1989b) analyzed word acquisition data from 45 middle- to upper-middle-class children studied between 13 and 20 months. Using the criterion of 60% common nouns for referential children at 20 months and less than 40% for expressive children, Hampson found four times as many expressive speakers (25) as referential (6). Indeed, if the cutoff of 25% or less common nouns is used for expressive speakers, one third of the sample (15) fall into this group. Nor was it the case that this variability among children was due to pace of vocabulary acquisition. Of 18 children identified as early talkers (15 or more words at 13 months), only 4 were identified as referen-

I I will not attempt to defend my own previous hypotheses (Nelson, 1985) regarding this shift. nor argue with Behrend’s. which would require more space than I can fairly take here. I do want to note that Dromi’s (1987) data have been used to support both sides of this debate. Like most other

careful observers (e.g., Piaget, Bowerman. Rescorla), she noted a number of complexive uses among her daughter’s first words.

Comment on “Constraints and Development” 333

tial at 13 and 20 months while 10 were expressive at 20 months by the criteria above.

Gentner (1982), whom Behrend relies upon for the counterclaim of vast consistency among children, examined noun predominance in early vocabularies among a variety of languages. Noun predominance is not the same as common noun or object class word predominance. For example, Gentner provided a table summarizing data from 8 of the children in my 1973 study, concluding that 68% of their first 10 words were nouns. However, only 42% of these words were common nouns, the others being proper names of people and pets. Moreover, the percentage of common nouns for individual children ranged from 22%-70%, reflecting the fact that 4 of the children were identified as expressive and 4 as referential. Similarly, Gentner included proper names among the nouns in her survey of different languages. If only common nouns are included, only the English and German samples reached the 50% level, the others ranging from 6% (Kahluli) to 47% (Japanese). (Even this count is liberal in that not all the common nouns learned refer to object classes, for example, “lunch” and “dinner.“) More recent research indicates that within-language variability of the kind observed among American children is also found among other language communities.

In the face of this documented variability among children, it is difficult to understand Behrend’s claim of “vast consistency.” It is true that when children learn nouns, they tend to learn the same ones: for example, “ball,” “dog,” “apple,” and so on. The obvious reason for this is that these are the things in their world with which they interact and that thus have importance for them (Brown, 1958; Nelson, 1973, 1974).

In his discussion of variability and individual differences, Behrend makes reference to the construct of range of reaction used by biologists, implying that variability in behavior around some central tendency is to be expected and is not cause for rejection of the constraint hypothesis. This is a perfectly reasonable point.2 However, the variability in vocabulary composition discussed above in terms of individual differences among children is of a different sort. When some children have as few as 6% nouns of their first words while others have as many as 80%, they clearly appear to be influenced by different constraints. The point is that constraints should apply to all children, not that they should be observable in all the behavior of all children all the time.3

Individual differences aside, it is disputable that children, in general, focus on object class .words from the beginning of word learning. For example, Gopnick

2 But it leaves open the question of whether 67% performance in an experiment is closer to the expected uniform-constrained-behavior or closer to chance plus bias.

3 It is not known whether individual differences arc observed in performance on the tasks that have been used in constraints experiments, but this is a possible explanation of some of the unex- plained variability in these experiments. Different children may be following different strategies.

334 Katherine Nelson

and Meltzoff (1986) argue-in contrast to Gentner (1982) and Behrend--that naming, far from being the predominant function of early words, is relatively rare in the early period. Although object names may predominate in (many) early vocabularies, they do not predominate in the child’s speech (see Bloom, 1973, for a similar view). Gopnick & Meltzoff emphasize the early acquisition and use of relational terms (e.g., “all gone,” “ no”) and suggest that children enter language using words for social purposes, move on to their use in cognitive plans, and only later (in the second half of the second year) focus on names as categorizors. This description is hard to reconcile with an object name constraint operating from the beginning of word acquisition as Behrend posits. Indeed, if an object name constraint were operating from the beginning, one would expect a much faster accumulation of vocabulary than is actually observed. Contrary to Behrend’s assumption, “fast mapping” does not apply to the first stage of word learning. Clearly, further research on the acquisition and function of words in this early period is needed to determine how accurate the Gopnick and Meltzoff proposal may be. Such research has to go beyond word counts to analyze the context in which words are naturally learned and used.

But even if we assume that noun learning is naturally predominant (and thus that non-noun learners arc deviant), that would not make the case for the word- object class constraint. There are many possible nonlinguistic reasons for leam- ing words referring to objects, including their perceptibility, frequency of the use of such words by parents, and frequency of such words in the language (even though articles and prepositions are more frequent tokens, noun types are the most frequent in conversations, as Gentner’s Table 11.6 shows). Gentner (1982) favors the “natural partition” hypothesis, that nouns and verbs have different conceptual bases. But this is not the same as an object name constraint in the child. It is rather a conceptual constraint on language. As Gentner (p. 323) notes: “a language is constrained by the nature of the perceptual world to make coherent lexicalization of objects.” There is all the difference between a con- straint on language and a constraint on the child. As stated previously, the child has certain perceptual and conceptual potentials, broad but ultimately limited. To be learnable, language must conform to those limitations, but those limitations are boundary conditions on humanly possible percepts and reasoning capacities, not specific linguistic constraints on the process of acquisition.

Behrend found my interpretation of constraints theories tendentious, but so far as I can see, his clarification of the claims accepts the same basic assumptions, namely, constraints are innate, universal, internal to the child, and specifically linguistic. These assumptions have strong implications that I attempted to draw out, namely, those of no change,4 no variation,5 inaccessibility to social influ-

4 Except through mnturation. But if maturation is invoked, the theory must specify when it kicks in and when it declines.

s Except for irrelevant reasons (noisy data). But note also that variation is to be expected from any process that has a biological basis. Perhaps the constraints theorists may wish to claim that those

Comment on “Constraints and Development” 335

ence, and not explainable in terms of general cognitive mechanisms. Some of the differences between Behrend’s discussion and mine inhere in his use of “con- straints” where I used (or intended) “a constraint.” Many constraints acting together might exhibit both change over time and variation within and among individuals in their effects, but a single constraint should operate in the same way whenever it is invoked. Thus far, there are single constraints (e.g., the taxonomic constraint, the mutual exclusivity constraint) that have been tested in experi- ments; therefore, the less than impressive percentages of behavior predicted from the constraint are relevant to the issue. A discussion of what other factors are operating to influence children’s behaviors in these experiments, including other possible constraints, constraints on constraints (e.g., explaining why they are not always invoked) are necessary to substantiate the claims.

Behrend further suggests that I am an implicit constraint theorist myself in proposing that the child comes to assume a one-to-one correspondence between words and concepts toward the middle of the second year, and in the assumption of intersubjectivity on the part of mother and child, which he views as a very strong constraint. As for the latter, it is only the necessary assumption for communication to take place. It is not an internal constraint, nor a necessarily linguistic assumption, but the condition of social interaction. As for the former, this is put forth as a developmental achievement by the child, and one that is ultimately wrong and must be abandoned, since conventional word meanings do not necessarily map directly onto the child’s (or anyone else’s) nonlinguistically based concepts.

1 regret if I gave the impression that I wished to dispense with experiments altogether. Experiments are among the most powerful of scientific tools, but they are not the only tools. Experiments are useful for modelling an observed phe- nomenon, and providing causal explanations, but to do so they must provide an accurate rendering of the phenomenon and test the hypothesized cause. The observed phenomenon here is that young children learn words. What is to be explained is frequently stated as the impressively fast and large vocabulary learned during the preschool period. One would expect that attention would be paid to the type of words the child acquired during this period and the natural conditions of their acquisition. As I have pointed out elsewhere (Nelson, in press) basic object class words are not the predominant type of word (or at least not the most challenging type) that the child is faced with during this period. The child has learned many object names by 2 years and easily learns others as needed. The real problem to be explained is how the child learns what he or she does: The many abstract social and cultural concepts that are incorporated into the language and presented in passing to children, who pick them up seemingly without effort. Even among the words known and used by some 20-month-olds,

(unfortunate) children whd do not learn predominantly nouns have a defective gene. At this point I

would not rule out a biological basis for individual differences in language acquisition, but I would bet against a word-object class constraint gene that was either present or not present.

336 Katherine Nelson

according to the Bates et al. (1988) inventory, there are a large number of nouns that do not refer to concrete objects, such as “bath,” “breakfast,” “nap,” “kitch- en, ” “rain,” “family,” “friend,” “help,” “home,” “day,” “morning,” “week,” “tomorrow,” and “uncle,” to list some at random.

Some of the recent experiments based on constraints hypotheses and dealing with taxonomic categories, inferences about natural kinds, and so on, have certainly revealed interesting data about children’s knowledge. But little has been learned about children’s acquisition processes by any of the experiments that I know of. Part of the reason, I believe, is that the only model that has been tested experimentally is the ostension model (i.e., point and name and see what the child does). This model, I have argued, is a minor part of word learning at any point in development, and is inapplicable to learning in the years between 2 and 6, and is clearly inappropriate to any but object name learning. Thus my complaint about the experiments with preschool children allegedly directed to mechanisms ex- plaining word learning is that they have not modelled the phenomenon and thus the proposed explanatory mechanisms have not been shown to apply to it.

Another, presumably primary, fact to be explained is that children can learn any words at all, given the indeterminate environmental conditions. One would expect that those interested in this question would first determine what the environmental conditions are and would attempt to show that those conditions could not explain the child’s learning. A second step would be to suggest alter- native explanations and to test them against each other. Instead, those who pose this question take its assumptions for granted, and ignore the vast amount of data, observational and experimental (e.g., Adams & Bullock, 1986; Lucariello, 1987; Lucariello & Nelson, 1986; Ninio 1980; Ninio & Bruner, 1978; Ross, Nelson, Wetstone, & Tanouye, 1986; Snow & Goldfield, 1983), that has been accumulated about the conditions of the child’s early acquisition of words (which is presumably where the major problem lies, since the more language available, the easier it is for the child to learn from language). Of particular significance is the Ninio (1980) study of mother’s use of ostension with 40 children in the second year (17-22 months), which found that in 95% of the cases in which mothers produced a word while pointing to a picture, the referent of the word was a whole object. Of the remaining cases, where the referent was part of another object, mothers made the relation clear by mentioning the whole as well as the part (as in “See the little girl’s shoe?“). A constraints theorist might want to claim that mothers have adjusted their naming practices to the constraints on the child, but it is at least equally likely that children learn from their mothers’ practices what kinds of referents are targets in ostensive teaching situations.

In reality, the constraints experiments are concerned with a different set of problems, namely, to delineate the biases that children have in applying novel words to possible referents. As noted previously, I believe that these experiments have produced some interesting results, and some-such as Markman and Hutchinson’s (1984) demonstration of differential picture choices for “same

Comment on “Constraints and Development” 337

kind” of named and not named objects-that are important to any explanation of conceptual and semantic development. However, as Behrend notes, the research claims to be about constraints on processes, not biases of behavior, yet there is no evidence for constraints other than the biased behaviors. That is to say, invoking constraints does not go beyond a description of the data.

A logical move to get beyond the demonstration of biases is to investigate their origin to determine whether they operate from the beginning of word learning or are the product of that process. Merriman and Bowman (1989) have undertaken this task with regard to the mutual exclusivity claim, finding that it develops by age 2% but that it is not a foundational constraint. Several similar investigations are apparently underway (e.g., Markman, 1989) and are to be applauded. Thus far the upshot of most of this work seems to be that some biases are present by 2 years but not much earlier. By 2 years, of course, the child has had over a year of experience with the uses of language. In her most recent statement Clark (1990) suggests that children must come to assume intentionality on the part of other speakers before they use pragmatic evidence for contrasts in the lexicon, thus tacitly endorsing a cognitive developmental basis for the con- trast constraint. Overall then, the evidence seems to be on the side of the devel- opment of biases during the word learning process, which is contrary to the assumption of a built-in linguistic constraint on the process.

The alternative to the linguistic constraint model is not, in my view, a simple social influence model, but a complex interactive functional model. Certainly I believe that the child makes a major contribution to his or her own learning, and that the state of his or her representational system is critical to what can and will be learned at any time. Nonetheless, the necessity for specific linguistic con- straints on the acquisition of a lexicon-beyond the perceptual and conceptual preparation, which all agree is in place by one year-is far from proved.

There are of course constraints on how the lexicon is ultimately composed, just as there are constraints on the possible composition of human diets. But the child who is acquiring knowledge of possible lunch foods is constrained by such general dietary factors in only the most indirect way. Sociocultural practices determine which of the indefinitely large number of foods possible for human digestion will appear on the family table. The child’s knowledge, thus, is con- strained by what his or her parents/caretakers offer. Nothing is learned in h vacuu’m, and what and how words (and foods) are presented to the child is as important as what the child’s cognitive (or digestive) state is. This all seems incontrovertible.

What is really mysterious is how the child with limited resources comes to understand so much of what is said. The kinds of things parents and teachers teach explicitly (e.g., object names, colors, shapes, numbers, the alphabet) are but the tip of the iceberg of accumulated knowledge that the 5-year-old pos- sesses. Although the 5-year-old is still woefully ignorant of many everyday concepts that we take for granted (as much research on taxonomic categories,

338 Katherine Nelson

kinship terms, and time, for example has demonstrated, nonetheless, the kinder- garten child has acquired a truly stunning vocabulary, only a small part of which is composed of concrete object terms. From this perspective it seems that con- straints theories have made a trivial problem into a mystery that requires power- ful constraints for its solution while ignoring the truly mysterious accomplish- ments of the young human mind.

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