The development of expressive language in young children with Down syndrome is reported to be severely retarded in comparison with typically developing children, although comprehension is well developed (Miller, 1987). However, there seem to be some discrepancies. For example, children with Down syndrome have rather advanced lexical development in comparison with other language domains such as grammar and semantic comprehension. It is reported that 20-to 36-month-old children with Down syndrome use the Novel Name-Nameless Category (N3C) principle to learn new words (Mervis & Bertrand, 1995). The N3C principle, or heuristic, refers to the way in which children “fast map” a new word to a basic level category. For example, a mother has bought her child a new set of toy animals comprising an elephant, a lion, a monkey and a giraffe. If the child is familiar with all the animals except the giraffe, and the mother says “where’s the giraffe?” the child will look for the animal for which she does not yet have a name. Mervis and Bertrand (1993, 1994, 1995) suggested that young children’s use of the N3C principle mainly contributes to lexical development. According to them, children use six different heuristics to learn new words: reference, extendability, object scope, categorical scope, the N3C principle, and conventionality.