Behavioral phenotype of individuals with Down syndrome

Evidence is reviewed for a developmentally-emerging behavioral phenotype in individuals with Down syndrome that includes significant delay in nonverba l cognitive development accompanied by additional, specific deficits in spe ech, language production, and auditory short-term memory in infancy and chi ldhood, but fewer adaptive behavior problems than individuals with other co gnitive disabilities. Evidence of dementia emerges for up to half the indiv iduals studied after age 50. Research issues affecting control group select ion in establishing phenotypic characteristics are discussed, as well as th e possible genetic mechanisms underlying variation in general cognitive del ay, specific language impairment, and adult dementia. (C) 2000 Wiley-Liss, Inc.