| Summary: | Language impairment is a key characteristic of many developmental disorders, with the relationship between linguistic and cognitive ability a critical topic for research in this field. Speech (articulation and phonology) and prosody have largely been absent from these discussions, perhaps because they are not universally impaired. The portfolio of published research critically appraised here addresses the relationships between speech and prosody and other domains, such as language and cognition, in two conditions in which disordered speech is common: primarily at the suprasegmental level in autism and at the segmental level in Down’s syndrome. Speech disorders were found in both conditions, though speech was much more severely impaired in Down’s syndrome. Errors were typically categorised as delayed phonological processes, implying a linguistic cause. However, through fine phonetic transcription and instrumental techniques it was shown that both conditions also presented with distortions that were more phonetic in nature and with non-developmental errors. Severity of speech disorder was not related to cognitive or linguistic ability as measured by standardised assessments, suggesting that a generalised delay in language or cognition was not the cause of disordered speech. In autism minor delays and distortions may be due to a lack of ability to identify with peers and impaired theory of mind, whereas in Down’s syndrome anatomical differences and difficulty with motor planning are likely causes. Both linguistic and paralinguistic prosody were found to be disordered in children with autism and correlations with linguistic ability were found. However, disordered prosody is more likely to be due to impaired theory of mind or weak central coherence than a result of delayed language. Both autism and Down’s syndrome present with speech that is disordered rather than simply delayed and this is unlikely to be due to delayed language, suggesting that specific, targeted intervention may be warranted.
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