Mean length of utterance (MLU) in morphemes was examined as a predictor of the grammatical complexity of natural language corpora of normal preschoolers and of children and adolescents with delayed language, Fragile X syndrome, Down syndrome, and autism. The Index of Productive Syntax (IPSyn) served as the measure of syntactic and morphological proficiency. For all groups, a strong curvilinear association between measures was found across the MLU range from 1.0 to about 4.5. Correlations were weaker when MLU exceeded 3.0 than during earlier stages of language development, however, confirming previous suggestions that MLU becomes less closely associated with grammatical development as linguistic proficiency increases. For the language-disordered groups, moreover, the curves relating the two measures differed from the curves for the normal preschoolers because MLU frequently overestimated actual IPSyn scores. The results are discussed with respect to the use of MLU in conjunction with other measures of syntactic complexity in the study of atypical language development.
Fragile X syndrome (FXS) is a well-recognized cause of mental retardation and developmental delay in males. Alongside the well-documented clinical characteristics of the condition, recent advances in technology and methodology have begun to define FXS at a number of different levels: genetic, brain structure and function, cognition, and behavior. This article suggests that the FXS phenotype is not merely a juxtaposition of spared and impaired functions but rather may be characterized by an inhibitory control deficit that interferes with the individual's ability to modulate output causing perseverative responding across various skill areas. It is further suggested that an inability to modulate arousal may be at least one cause for the inhibitory control deficit that typifies the FXS phenotype. The approach to understanding atypical development outlined here holds exciting promise for future research in FXS and other developmental disorders.
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