平均発話長(MLU)から捉えた日本語の初期文法発達—自立語付属語MLUを指標として

Measuring early Japanese grammar acquisition with mean length of utterance(MLU): Use of MLU analysis based on independent words and particles

　自立語付属語を単位とした平均発話長(MLUw)の初期文法発達の指標としての有用性を検討するために，5名の定型発達幼児の発話データベースを用いて，項どうしや項と述語をつなぐ文法的助詞および動詞活用形の種類の増加とMLUwの上昇との対応関係を分析した．MLUw 2.0までは，MLUwの増加に伴い文法的助詞と動詞活用形の多様化が認められ，MLUwが文法発達速度の個人差と対応していたことからもMLUwは初期の文法発達指標となりうることが示唆された．また，5児に共通して獲得された9種類の文法的助詞および11種類の動詞活用形の獲得時期とMLUwから，最初の動詞活用形が生産的になる文法的萌芽期，多くの文法的助詞や動詞活用形が出現し柔軟に使用される文法的拡張期，さらに獲得が進展し徐々にプラトーに近づく文法的発展期の3段階に分けられることが示唆された．MLUw 2.0以降は文法指標との関連性は失われていった．

　This study analyzed the speech database of 5 typically developing children and examined the possibility of using Mean Length of Utterance Measured in Words (i.e. contents words and particles, hereafter MLUw) as an index for early grammar acquisition. Up to MLUw 2.0, the increase in MLUw corresponded to the increase in the variety of grammatical particles and verb inflection forms, and it also corresponded to individual differences in the rate of grammatical development, suggesting that MLUw can be used as an index for early grammar acquisition. Based on the acquisition order of 9 particles and 11 inflection forms acquired by all 5 children, we identified the following 3 developmental stages: "the period of emerging grammar", where the first inflections were acquired and became productive, "the period of extended grammar", where a large number of grammatical particles and verb inflection forms were used flexibly, and "the period of further grammatical growth", where the acquisition progressed approaching a plateau. Beyond MLUw 2.0, the correspondence between MLUw and further expansion of grammatical particles and verb inflection forms decreased sharply, resulting in a low reliability of MLUw.