Mental retardation is defined by the American Association on Mental Retardation as significantly subaverage intellectual functions with related limitations in social and behavioral skills. According to the most recent estimates (Larson et al., 2001), the prevalence of mental retardation in the noninstitutionalized population of the United States is 7.8 people per thousand; if institutionalized individuals are included in the prevalence rates, the number increases to 8.73 per thousand. Mental retardation is associated with limitations in learning and in the ability to communicate, and has a profound effect on a child's ability to learn to talk. At one time it was believed that the language acquisition of all persons with mental retardation represented a slow-motion version of normal language development. This hypothesis has two major flaws: first, patterns of language development vary across types of mental retardation, and second, within a single type of mental retardation, there is considerable heterogeneity.

The majority of research on children with mental retardation has involved children with Down syndrome (or trisomy 21). This syndrome is the most common genetic cause of mental retardation, occurring in approximately one out of every 800 births. Because Down syndrome is identifiable at birth, researchers have been able to trace developmental patterns from the first months of life. The development of speech and language is severely affected in children with Down syndrome, with levels lower than would be expected, given mental age (Miller, 1988). Speech intelligibility is compromised throughout the life span because of problems with articulation, prosody, and voice.

Children with Down syndrome differ from the normal population in respect to a variety of anatomical and physiological features that may affect speech production. These features include differences in the vocal cords, the presence of a high palatal vault and a larger than normal tongue in relation to the oral cavity, weak facial muscles, and general hypotonicity. Although the precise effect of these differences is difficult to determine, they undoubtedly influence speech-motor development and thus the articulatory and phonatory abilities of children with Down syndrome. An additional factor affecting the speech of children with Down syndrome is fluctuating hearing loss associated with otitis media and middle ear pathologies.

Fragile X syndrome, the most common known cause of inherited mental retardation (Down syndrome is more common but is not inherited), has an estimated prevalence of approximately one per 1250 in males and one per 2500 in females, with males exhibiting more severe effects. Little research has been done on the speech of young children with fragile X syndrome. Available reports on older children (Abbeduto and Hagerman, 1997) indicate abnormalities in articulatory development, disfluncies, and the presence of atypical rate and rhythm. These abnormalities may be attributed, in part, to differences in the structure and function of the oral-motor systems of boys with fragile X syndrome, including excessive drooling, hypotonia involving the oral-facial muscles, and the presence of a narrow, high-arched palate. Like their peers with Down syndrome, children with fragile X syndrome have a high incidence of otitis media and intermittent hearing loss.

Autism is a developmental disorder with prevalence estimates ranging from two to five per 10,000 (3:1 males). This disorder is characterized by deficits in social interaction, communication, and play; two out of three children with autism are mentally retarded (Pennington and Bennetto, 1998). Although in phonetic form, the prelinguistic vocalizations are like those of nonretarded infants, social communication skills in the prelinguistic period are atypical. About 50% of autistic children fail to develop spoken language; the other 50% exhibit delays in acquiring language, although not to the same extent as children with Down syndrome do. Speech production is characterized by echolalia and abnormal prosody (see autism).

Williams syndrome, a genetic disorder that includes mental retardation, is relatively rare, occurring in one in 25,000 live births. One of the most striking aspects of Williams syndrome is that, in spite of marked impairments in cognition, linguistic skills appear to be relatively normal (Bellugi, Lai, and Wang, 1997; Mervis and Bertrand, 1997). This dissociation of language and cognition underscores the importance of examining the relationship between mental retardation and speech in a variety of mentally retarded populations.

The foundations for speech development are laid in the first year of life, with the emergence of nonmeaningful vocal types that serve as precursors for the production of words and phrases. Of particular importance is the production of consonant-vowel syllables, such as [baba], which generally appear around age 6–7 months. Phonetically, these “canonical” babbles are similar or even identical to the forms used in first words; thus, the production [mama] may be a nonmeaningful babble at 8 months and a word at 14 months. The difference is recognition of the sound-meaning relationships that are the basis for words. In general, prelinguistic vocal development of infants with mental retardation resembles that of their nonretarded peers in terms of types of vocalizations and schedule for emergence. Infants with retardation begin to produce canonical babble within the normal time frame or with minor delays.

Despite the nearly normal onset of canonical babble, however, the emergence of words is often delayed among infants with mental retardation, particularly those with Down syndrome (Stoel-Gammon, 1997). Research suggests great variability among children in this domain, with a few reports of word use in the second year of life for a few children with Down syndrome but the majority showing first words appearing between 30 and 60 months. The magnitude of the delay cannot be easily predicted from the degree of retardation. Moreover, once words appear, vocabulary growth is relatively slow. Whereas nonretarded children have a vocabulary of 250 words at 24 months, this milestone is not reached until the age of 4–6 years for most children with Down syndrome.

In terms of phonemic development, acquisition patterns for children with mental retardation are similar to those documented for nonretarded children (Rondal and Edwards, 1997). In the early stages, words are “simplified” in terms of their structure: consonant clusters are reduced to single consonants, unstressed syllables are deleted, and consonants at the ends of words may be omitted. Phonemes that are later-acquired in normal populations, primarily fricatives, affricates, and liquids, also pose difficulties for children with mental retardation. Among nonretarded children acquiring English, correct pronunciation of all phonemes is achieved by the age of 8 years. Some reports suggest that the phonologies of children with Williams syndrome may be relatively adultlike by the (chronological) age of 8.

In contrast, individuals with Down syndrome, even when they have a mental age of 8, exhibit many articulation errors. Moreover, comparisons of phonological development in three populations matched for mental age, Down syndrome, non-Down syndrome with mental retardation, and typically developing, revealed a greater number and variety of error types in the children with Down syndrome (Dodd, 1976). A persistent problem in children with Down syndrome, is that their speech is hard to understand (Kumin, 1994). Parents report low levels of intelligibility through adolescence as a result of speech sound errors, rate of speech, disfluencies, abnormal voice quality, and unusual voice quality. There is some indication that children with fragile X syndrome also suffer from low levels of intelligibility (Abbeduto and Hagerman, 1997).

For many children with mental retardation, delays in the acquisition of speech and language may serve as the first indication of a cognitive delay (except for Down syndrome, which is easily diagnosed at birth). Parents may be the first to raise concerns about atypical patterns of development, and pediatricians and social workers should be aware of the link between linguistic and cognitive development. Once mental retardation has been confirmed, assessment typically adheres to traditional practices in speech-language pathology. In the prelinguistic period, which may be quite protracted for some children, assessment is initially based on unstructured observations and parental report. If language is slow to emerge, it is important to assess hearing and oral-motor function. More formal assessments are done in two ways: by means of standardized tests that focus on the individual sounds and structures of a predetermined set of words (i.e., a normed articulation test) and by analyzing samples of conversational speech to determine intelligibility and overall speech characteristics.

Recommendations for the treatment of speech deficits in children with mental retardation range from intervention directed toward underlying causes such as hearing loss and deficits in speech-motor skills (Yarter, 1980) to programs aimed at modifying parent-to-child speech in order to provide optimal input in the face of delayed language acquisition. Most phonological interventions focus on increasing the phonetic repertoire and reducing the number of errors, using therapy techniques similar to those for children with phonological delay or disorder. In some cases, therapy may occur at home, with the parents, as well as in the clinic (Dodd and Leahy, 1989; Cholmain, 1994).

See also communication skills of people with down syndrome; mental retardation.