4. The Domain of the Conscious Grammar: The Morpheme Studies
The Morpheme Studies: A brief history
The study of grammatical morphemes has been particularly fruitful for understanding the mechanisms involved in second language acquisition by adults. Aside from merely telling us in what order certain structures are in fact acquired, these studies have also been of value in revealing the domain of the acquired and learned grammars, when performers appeal to conscious learning and when they do not.
The history of morpheme studies in language acquisition begins with Brown's demonstration (Brown, 1973) that children acquiring English as a first language show a similar order of acquisition for grammatical morphemes in obligatory occasions. Certain morphemes, such as ing and plural, tend to be acquired relatively early, while others, such as the third person singular /s/ on verbs in the present tense (III sing.) or the possessive 's marker tend to be acquired late. Brown's longitudinal findings were confirmed cross-sectionally be de Villiers and de Villiers (1973). This discovery was extended to child second language acquisition by Dulay and Burt (1973, 1974a, 1975) in several cross-sectional studies, by Kessler and Idar (1977), and by Rosansky (1976) (who has a rather different view of her results, to be discussed below). The child second language order was not identical to the child first language order, but there were clear similarities among second language acquirers. (As I have pointed out in several places (Krashen, 1977a; Krashen, Butler, Birnbaum, and Robertson, 1978), this appears to be due to differences in the rank order of free morphemes, especially copula and auxiliary, which tend to come later for the first language acquirer.)
The discovery of the "natural order" has allowed us to probe the interaction between language acquisition and language learning in the