Three kinds of evidence point to the initial priority of
names for things over labels for the attributes of those same
things. The first concerns the kinds of words that comprise early
productive vocabularies. Nouns dominate; dimensional adjectives
are rare or non-existent. For example, in Stern's
diary study of the acquisition of English
[Gen78],
78% of the words produced
at 20 months were nouns while none were adjectives. Similarly, in
Nelson's
[Nel73]
study of 18 children learning English, fewer than 7% of the
first 50 words were adjectives.
The priority
of nouns over adjectives in early vocabularies is evident in other
languages as well. In Dromi's
[Dro87]
study of one child learning Hebrew,
only 4 of the first 337 words were adjectives. In a longitudinal
study of the acquisition of Spanish by 328 children, Jackson-Maldonado
et al.
[JMTM
93]
found only one adjective among the 88 most common words. The
finding that adjectives are infrequent in early vocabularies is
remarkable given that common dimensional adjectives such as size and
color terms are among the most frequently used words in adult
language.
The second class of evidence concerns studies of artificial word learning. In this commonly used method, experimenters present a novel object to a child and label it with a novel word (e.g., ``this is a dax''). Children's interpretation of the word is measured by the kinds of other objects to which they generalize the newly learned label. Considerable evidence indicates that by 18 months (and quite possibly before), children interpret novel nouns as referring to taxonomic categories [Mar89,Wax94]. Further, the evidence suggests that children remember what they have learned over several days and weeks [WMF94]. There have been a number of attempts to use these methods to teach novel adjectives. In these studies, the novel word is placed in an adjectival context (e.g., ``this is a daxy one'') or is explicitly contrasted with a known adjective (e.g., ``this is ecru, not red''). Learning in these instances has proved modest at best, even in children as old as 36 months [AL90,AM87,Car78,SJL92,TG88]. Cross-linguistic studies of artificial word learning also suggest that names for concrete things are special in early language learning [IG93,Wax94] in that there are considerable similarities in the nature of children's noun extensions across languages and considerable variability across (and within) languages in young children's interpretation of novel adjectives. Other evidence from children learning English suggests that the initial meanings of dimensional terms may be highly context specific [KC80]. In sum, whereas names for things appear to be ``fast mapped'' [Car82] to potential categories, the extension of a novel adjective appears more slowly and more variably determined.
The third class of evidence concerns children's errors with nominal and adjectival meanings. There are extensive literatures in both areas although they are difficult to compare because of vastly different methods, ages of subjects, and empirical questions asked. These differences derive directly from the noun advantage over adjectives. The key question for researchers who study early noun acquisition is how it is that children learn so many nouns so rapidly and with so few errors. The only errors consistently studied in this literature are the overextension errors typically noticed at about the time productive vocabulary first begins to accelerate. However, there is a debate as to whether these errors are category errors. Instead, these overextensions (for example, calling a zebra ``doggy'') may reflect pragmatic strategies or retrieval errors [GSS96,Hut74]. Consistent with this idea is the rarity of overextensions in comprehension (see, for example, Naigles & Gelman, 1995).
In contrast, the key question for researchers who study the acquisition of dimensional adjectives is why they are so difficult to learn. The central phenomena are comprehension errors. Long after children begin to use dimensional words, when they are as old as 3, 4, or even 5 years, their interpretations of dimensional adjectives are still errorful. This literature is replete with examples of both within- and between-dimension errors, interpreting big to mean TALL [Mar88], big to mean BRIGHT [Car78,Car82], dark to mean LOUD [SS92], and blue to mean GREEN [BS93]. Although plentiful, these errors are constrained. They consist of confusions within the semantic domain of dimensional terms. That is, children may confuse dark and loud but they do not confuse dark and room. The category specificity of these errors means that at the same time children are rapidly learning nouns and commonly misinterpreting adjectives, they have some idea that nouns and adjectives span different categories of meaning.
In sum, the phenomena to be explained are (1) why common nouns are acquired by young children earlier, more rapidly, and with fewer errors than are dimensional adjectives and (2) how, during the protracted course of learning dimensional adjectives, young children seem to recognize that the dimensional adjectives comprise a class.