Shortly after Brown's results were published, Dulay and Burt (1974, 1975) reported that children acquiring English as a second language also show a "natural order" for grammatical morphemes, regardless of their first language.
(For a review, see Dulay, Burt, and Krashen, in press. Ravem, 1974; Milon, 1974; Gillis and Weber, 1976; Cancino, Rosansky, and Schumann, 1974; Wode, 1978 and Nelson, 1980 are some second language studies in this area.) There is surprising uniformity here as well--acquirers make very similar errors, termed developmental errors, while they are acquiring.
Observation 4: Learners’ output (speech) often follows predictable paths with predictable stages in the acquisition of a given structure. Learners’ speech shows evidence of what are called “developmental sequences.” One example involves the acquisition of negation in English. Learners from all language backgrounds show evidence of the following stages: Stage 1: no + phrase: No want that. Stage 2: subject + no + phrase: He no want that. Stage 3: don’t, can’t, not may alternate with no: He can’t/don’t/not want that. Stage 4: Negation is attached to modal verbs: He can’t do that. Stage 5: Negation is attached to auxiliaries: He doesn’t want that.