Roger Brown’s (1973) First Language Development Study and MLU LIN Wen-Zhi Southwest University

Brown (1973), in his seminal work A First Language: The Early Stages, suggested that instead of using average number of words, syllables or age to index language development，it would be more useful to measure the Mean Length of Utterance in terms of morphemes (MLU).

Morphemes can be both bound and unbound Morphemes can be both bound and unbound. For example in the sentence, I wanted to eat the cookies, there are 8 morphemes, 2 bound morphemes and 6 unbound morphemes. Note the difference in complexity of the sentence when it contains the inflectional morphemes, and that the bound morphemes assist the listener to understand that the desire for cookies was in the past and that more than one cookies were wanted.

The learner who informs the listener that his "want" was an event in the past (ed) related to more than one cookie (s) is a more sophisticated speaker and therefore demonstrates, for the psycholinguist, more advanced application of the "rules of language” by the child. Consequently, Brown concluded that MLU is “an excellent simple index of grammatical development because every new kind of knowledge increases length….”(1973 ， p.53)

Beyond about 5 or 6 years of age, given the wide variety of sentence constructions of children, MLU loses its value in measuring knowledge and complexity. Context and type of interaction then determine the complexity. Brown's (1973) research culminated in the development of a five-stage framework to understand typical language development according to the rules of grammar related to syntax and morphology.

The stages are not based on chronological age (since children vary greatly in speed of language acquisition), but are based on the mean length of utterance (MLU). Each stage is referenced to MLU as the index of the progression of language complexity through morpheme combining.

Brown identified 14 different obligatory grammatical morphemes that he used as markers of the progression of language complexity across his stages 2-5. Some examples are "in" as a preposition, plurals, past tense, possessives, contractions, articles, etc.

Roger Brown's Stages of Language Development Stage 1: 15-30 Months, MLU 1.75 (two-word stage after 50-60 single word utterances) Stage 2: 28-36 Months, MLU 2.25 Stage 3: 36-42 Months, MLU 2.75 Stage 4: 40-46 Months, MLU 3.50 Stage 5: 42-53 Months, MLU 4.00 Note. Stage I begins as soon as the MLU goes above 1.0 (or multi-word utterances) and ends at 2.0, Stage II goes from 2.0 to 2.50.

Stage 1 speech is characterized as telegraphic. The words that occur in Stage 1 refer mainly to objects, people, actions, in the environment. These words are mainly content words. Agent-action or Agent-Object sequence is the typical form.

During Stage 1 the meaning of the language may be obscured because the child is not using unbound morphemes such as articles, auxiliary verbs (is, has), irregular verb tenses, negation (not that one), conjunctions, etc. Moreover， the "vocabulary" of the learner is limited as well. In addition, bound morpheme markers related to tense and plurality, etc., are absent from the speech production of 2.5 year old children.

It isn't until Stage 2 that grammatical morphemes emerge. For Brown, it is not until the child is about 2.5 - 3 years old that language that "modulates the meaning" begins to develop. The child begins to use more functor or function- words.

In Stage 2, the development centers on the modification of meaning through the acquisition of grammatical morphemes. Brown presents a detailed study of the grammar and semantics of 14 morphemes and suggests that the order of their acquisition is dependent on the relative grammatical or semantic complexity. These morphemes add number, tense, aspect, specificity or non-specificity, etc., to the naming words of Stage I, nouns and verbs.

During the second stage and then those stages that follow the child begins to produce combinations of bound and unbound morphemes that support understanding by the listener. These are all words，phrases and inflections (endings) that cannot occur without other content words in order to convey meaning.

The concept of MLU and the rules for calculating it were developed in order to compare language acquisition between different children (or the same child at different developmental stages) , and hopefully , between different languages. Brown claims that MLU and the rules he gives serve “as a simple way of making... development in one language comparable with development in another“ (p.54). Brown suggests that these five stages are universal.

Roger Brown’s (1973) book, a major contribution to research on child language, describes the first two of the five sequential stages; a later work will deal with the remaining three stages. The book is concerned with sentence construction, meaning and understanding, and “it completely excludes pronunciation and the growth of vocabulary”.