4 Chapter 1
Several factors must be considered in the diagnosis of intellectual disability. This diagnosis
requires t he indiv idua l ’s intelligence quotient to fa ll t wo or more standard dev iations below t he mean,
the condition must occur during the developmental period, and it must impair adaptive be hav iors.
The most controversial aspect of the diagnosis of intellectual disability is intelligence testing.
The intelligence quotient can be compared to other mental test scores, and levels of intellectual
disability can be determined. Individuals with mild and moderate intellectual disability are some-
times labeled borderline and educable, respectively, and they either live and work in de pen dently
or have some experience in sheltered environments. These individuals usually have the mental
capacity to acquire language. Persons with more severe intellectual disability, sometimes labeled
trainable or severe and custodial or profound, are usually unable to acquire language, although
individuals in these categories may have some potential to learn it.
Linguistic Development
The linguistic development of children has been studied for de cades, and today we have a good
understanding of the stages children pass through in becoming linguistically competent. By study-
ing individuals and large groups of children, scientists have learned when specific grammatical,
phonological, and syntactic pro cesses are acquired. Beginning with the prelinguistic be hav ior of
undifferentiated crying and ending with the acquisition of high- level abilities to use and under-
stand grammatical inf lections, norms have been created that determine when children achieve
these milestones. Although we refine our linguistic competence and per for mance throughout our
lives, by about the age of 6 or 7 years, the structure and foundation of language have been learned
(Tanner et al., 1997). Owens (2001) notes that language development is most remarkable in the first
5 years, when 90% of the syntax, morphology, and phonology are learned.
Children acquire the structure and form of language during a period of rapid learning. As
noted earlier, the pro cess begins with undifferentiated crying, in which the child’s cries are ran-
dom and little information is expressed. During the first month, crying soon becomes differenti-
ated, and the child’s parents can discern dif fer ent types of cries for dif fer ent needs. Studies have
shown that mothers can understand six or seven dif fer ent cries in their infants— for example, for
pain, hunger, and the desire to be held. Children continue their linguistic development with coo-
ing, babbling, and combining one, two, and three or more words into longer utterances. The child
learns to use and understand linguistic constructs such as those for rejection, notice, possession,
cessation, disappearance, and so forth. Grammatical, phonological, and syntactic pro cesses are
gradually learned and refined, and by school age, basic linguistic competence and per for mance
have been achieved.
Linguistic disorders can be displayed in four ways. First, some children show a relatively
uniform pattern of delay across all aspects of linguistic development. Usually because of global
intellectual disability, their general cognitive development is arrested, and their linguistic abilities
ref lect the delay. These children have relatively consistent reductions in all aspects of linguistic
development. For example, a child with a chronological age of 6 years will use the grammatical,
syntactic, and phonological pro cesses of a 3- year- old child. Second, some children have a defi-
ciency in a par tic u lar aspect of linguistic development, whereas other aspects are normal. For
example, a child may only have prob lems with self- expression in writing. Third, a child may expe-
rience overall delay in linguistic development, but one or more aspects will be disproportionately
impaired. Fi nally, some children have delayed or disordered linguistic development, but one or
more aspects will be highly developed and functional, such as seen in savants. Table 1-1 shows the
manifestations of linguistic disorders.