Locke describes these as possible differences among people, but for our purposes this description can be viewed as the acquisition of the meaning of "gold" by one person. This can be traced by the following progression of the acquisition of semantic features:
It is of interest that this acquisition proceeds only by the addition of components. Since each additional. component serves to further narrow the meaning, this progression moves from a general meaning, which is over-extended, to a more specific one. Sometimes children also attach incorrect additional features to the meaning of a word. For example, suppose the child's first exposure to the word "gold" was in relation to gold coins. He might then come to attach the feature circular to "gold" in addition to whatever other features he has acquired. This would result in "under-extension." The child's meaning would be too specific and there would be gold things he would not call "gold." Piaget (1928) finds that many children think the term "brother" can refer only to children, not to adults. This is another example of under-extension.
This phenomenon is not as noticeable as over-extension, since over-extension results in children applying names to incorrect referents, while under-extension generally just results in the child not naming something; mistakes are more obvious than omissions. However, it seems to be true that word meaning acquisition more often goes from general to specific meanings, rather than vice versa.
Locke (1690) believed that the basic units forming the ideas expressed by words have some sort of psychological primitiveness. He referred to these as "simple ideas" and believed that these could be recognized by the pre linguistic child. Those categories of features E. Clark describes (shape, size, taste, etc.) are clearly derived from the child's perception of the properties of the objects in his environment. These are further discussed in E. Clark (1973b). Bierwisch agrees and writes of the minimal units of meaning he calls components:
They are not symbols for physical properties and relations outside the human organism, hut rather for the internal mechanisms by means of which such phenomena are perceived and conceptualized. This leads to the extremely far-reaching, though plausible, hypothesis that all semantic structures might finally be reduced to components representing the basic dispositions of the cognitive and perceptual structure of the human organism. According to this hypothesis semantic features cannot be different from language to language, but are rather part of the general human capacity for language. (1970, pp. 181-182)