Similar conclusions follow from studies of two different genetic disorders – Specific Language Impairment (SLI) and Williams Syndrome – that affect language in different ways. Individuals with SLI are generally of normal intelligence and have no hearing impairment. But they are slow to produce and understand language, and their speech is characterized by the omission of various inflectional morphemes. Individuals with Williams Syndrome have a genetic disorder linked to various heart problems, elevated levels of calcium in their blood, and a characteristic appearance (short stature, an upturned nose, a long neck, among other things). Their language and social skills are in the normal range, but in other respects such as motor control and spatial perception they display mild or moderate developmental delay.

What is significant for our purposes is that these disorders provide more evidence for the organization of our mental lexicon. Individuals with SLI find it difficult to create the past tenses of novel verbs, and often fail to inflect unfamiliar regular verbs correctly; they have less difficulty with irregular verbs, though. In spontaneous speech, they may leave the regular past tense off verbs (Redmond and Rice 2001). In contrast, individuals with Williams Syndrome speak fluently and produce sentences with correct regular past tenses, but have more trouble with irregular ones; indeed they seem to use regular past tense marking even where control subjects or individuals with SLI would not, for example, overgeneralizing the regular -ed ending on irregular verbs (for example, falled) (Clahsen, Ring, and Temple 2004). Assuming that the genetic anomalies associated with these disorders affect different parts of the brain, we can explain this pattern of behavior.