A longitudinal study of children with developmental language delay at age three: later intelligence, reading and behaviour problems.
A large sample of Dunedin (New Zealand) children were assessed at age three to identify those with language delay. 2.6 per cent were defined as delayed in verbal comprehension only, 2.3 per cent as delayed in verbal expression only, and 2.3 per cent as delayed in both ('general language delay'). Most of these children, and the remainder of the sample, were assessed for intelligence, reading and behaviour problems at ages seven, nine and 11. Those in every language-delay group had significantly lower mean IQs and lower mean reading scores than the remainder of the sample. They also more often had a low IQ or a lower reading score at ages seven and nine and a lower Verbal and Full-scale IQ at 11. The groups with delayed verbal comprehension and general language delay had significantly more behaviour problems than the remainder of the sample. The group with general language delay was consistently the most disadvantaged in later intelligence, reading and behaviour. Two of the language-delay groups (comprehension and general language) had significantly higher scores on a family disadvantage index. The results of this study confirm the importance of early language delay as a predictor of lower than average intelligence and reading ability and increased behaviour problems.