prehending language messages. Language skills  need to be assessed in both expressive
and receptive modalities as these may differ in severity. For example, an individual’s ex-
pressive language may be seve rely impaired, while his recept ive language is hardly im-
paired at all.
Language disorder usually affects vocabulary and grammar, and these effects then
limit the capacity for discourse. The child’s fi rst words and phrases are likely to be delayed
in onset; vocabulary size is smaller and less varied than expected; and sentences are
shorter and less complex with grammatical errors , especially in past tense. Deficits in com-
prehension of language are fr equently underestimated, as children may be good at using
context to infer meaning. There may be word-finding problems, impoverished verbal def-
initions, or poor understand ing of synonyms, multiple meanings, or word play appro-
priate for age and culture. Problems with  remembering new words and sentences are
manifested by difficulties following instructions of increasing length, difficulties rehears-
ing strings of verbal information (e.g., remembering a phone number or a shopping list),
and difficulties remembering novel sound se quences, a skill that may be important for
learning new words. Difficulties with discour se are shown by a reduced ability to provide
adequate information abou t the key events and to narrate a coherent story.
The language difficulty is manifest by abilities substantially and quantifiably below
that expected for age and significantly inte rfering with academic achievement, occupa-
tional performance, effective communication, or  socialization (Criterion B). A diagnosis of
language disorder is made based on the synthesis of the individual’s history, direct clinical
observation in different contexts (i.e., home, school, or work), and scores from standard-
ized tests of language ability that can be  used to guide estimates of severity.