Like oral and written languages, American sign Language (ASI) makes use of grammatical markers to indicate such things as time (for example, past tense) and number. These markers are expressed through specific hand or body movements
Elissa Newport (1990) and her colleagues studied the ability of deaf users of ASL to produce and comprehend grammatical markers. They compared Native signers (who were exposed to ASL from birth), Early signers (who began using ASL between four and six years of age), and Late signers (who began learning ASL after age 12). They found no difference between the groups in some aspects of their use of ASL, for example in vocabulary knowledge. However, on tests focusing on grammatical markers, the Native group used the markers more consistently than the Early group who, in turn, used them more consistently than the Late group.