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

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

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.

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.

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.