Taxometric analyses of specific language impairment in 6-year-old children.

Purpose: To determine whether language scores at age 6 years suggest that specific language impairments (SLIs) distribute in a categorical or in a dimensional fashion.

Method: A taxometric analysis of language scores from 601 six-year-old children who were free of neonatal risk factors was performed. From among 4 candidate indicators of SLI, 2 were eligible for the mean above minus below a cut (MAMBAC) procedure (Meehl & Yonce, 1994): number of different words (NDW) produced in a language sample and average percentage phonemes repeated correctly in 3- and 4-syllable nonwords (3-4 PPC). Graphs of these MAMBAC variables were inspected to determine whether they patterned in a manner suggesting the presence of a discrete category, having either a central peak or a steep curve peaking near the final interval.

Results: MAMBAC curves for NDW and 3-4 PPC did not suggest the presence of a taxon, and results did not vary during consistency checks in which MAMBAC parameters and cutoff values for language deficits were manipulated.

Conclusions: Results suggest that for these measures, children with specific language deficits are those falling at the lower end of a continuous distribution of language skills rather than a qualitatively distinct group.