One way of construing the problem is in terms of category learning. Why are common noun categories more easily learned than common adjective categories?
Several proposals have been offered suggesting a foundational conceptual distinction between objects and their attributes. For example, [Gen78], [Mar88], and [Mac82] have all suggested that nouns are logically prior. They point out that predicates presuppose arguments but that the reverse is not true. The suggestion, then, is that children need not understand shaggy to figure out what dog means from examples like the dog is shaggy but must know dog to figure out shaggy from the same sentence. Similarly, Markman (1989; see also, Carey, 1994)
proposed that children's initial hypotheses about word meanings adhere to a ``whole-object principle'' --- that children assume that novel labels refer to individual whole objects rather than to their component properties or to collections of objects. Thus, by this account, children's initial hypotheses about meanings are noun-like. Although these proposals are probably somewhat correct, they seriously underspecify the processes through which knowledge about the differences between nouns and adjectives is instantiated or acquired.
We seek such specification in a similarity-based account. Our idea is that the noun advantage and an initial segregation of nouns and adjectives as distinct classes of words is the result of the most general and ordinary processes of associative learning. There are two arguments for this approach which we find compelling. First, whatever else children know or believe, similarity-based associative learning is part of their biology and thus a good place to begin looking for a mechanistic account. Second, similarity-based learning would seem crucial at the front-end when children know no language. At this point, children learn many words by ostensive definition [Mer87]. Parents point to an object and say, for example, ``that's a dog'' or ``that's big.'' This associative task of mapping words to perceptible properties would seem to be the very same for the learning of dimensional adjectives as for the learning of nouns. Even if the child possessed some pre-existing conceptual distinction between objects and their properties, the child could not use that knowledge at this stage because the child has no words and thus no knowledge of the syntactic frames that would distinguish whether a novel word is a noun or an adjective. In the beginning, the young child can only associate novel labels with the properties of things so labeled. Doing so will yield a representation of dog as things with DOG properties and a representation of wet as things with WET properties. While incomplete, such meanings are in fact on the right track.
Given these assumptions, we ask: Why are common nouns learned more readily than common adjectives?