One of the hallmarks of SLI is a delay or deficit in the use of auxiliary verbs (e.g., be, do) and other grammatical morphology (e.g., third person singular -s, past tense -ed). They omit function morphemes from their speech long after age-matched children with typical language development show consistent production of these elements.
Some researchers claim that SLI children's difficulty with grammatical morphology is due to delays or difficulty in acquiring a specific underlying linguistic mechanism. For example, Mabel Rice and Ken Wexler suggest that children with SLI have difficulty acquiring the rule that verbs must be marked for tense and number.
A second hypothesis is that these children have a deficit in processing brief and/or rapidly- changing auditory information, and/or in remembering the temporal order of auditory information. For example, Paula Tallal has found that some children with SLI have difficulty reporting the order of two sounds when these sounds are brief in duration and presented rapidly. Laurence Leonard suggests that these deficits may underlie difficulties in perceiving grammatical forms (e.g., "the", "is"), which are generally brief in duratio.
A third hypothesis is that children have poor short-term memory for speech sounds. Children with SLI perform worse than children with typical language skills on repeating nonsense words (for example, "zapanthakis"). In a number of recent studies short-term memory for speech sounds has been shown to correlate highly with vocabulary acquisition and speech production. This has led to the hypothesis that a primary function of this memory is to facilitate language learning.
A single nucleotide polymorphism (rs17236239) in a gene called CNTNAP2 may be associated with SLI.