Language acquisition of same-sex, multiple-birth siblings: A nature/nurture study

Abstract

This retrospective study considers the extent to which nature and nurture influence early first language acquisition. Data sets that address rate of grammatical acquisition, acquisition of regional dialect features, and temperament are examined. The subject population consists of four multiple-birth, same-sex siblings born to a European-American, college-educated couple living in the South. Two of the siblings are monozygotic (identical DNA), and the other two are dyzygotic (at least 50% shared DNA). Pre-existing audio tapes recorded when the children were ages 2 through 6 were examined using Mean Length of Utterance (Brown, 1973) and Developmental Sentence Structure (Lee, 1974) to determine grammatical development. Tapes also were examined for acquisition of two regional phonological features: monophthongization of /ai/ and variants of (ing), and two lexico-syntactic features, yall and fixin to. Carey Temperament Scales, completed by the parents, were used as a third estimate of similarity or difference. Although the retrospective nature of the study limits the data available for analysis, results show that the monozygotic siblings are more similar to one another in both grammatical acquisition and regional forms than are the dyzygotic siblings. Acquisitional differences appear more pronounced at earlier ages, indicating that genetics may be more influential in earlier years.