Silence is liberating: Removing the handcuffs on grammatical expression in the manual modality

Grammatical properties are found in conventional sign languages ofthe deaf and in unconventional gesture systems created by deaf children lacking language models. However, they do not arise in spontaneous gestures produced along with speech. The authors propose a model explaining when the manual modality will assume grammatical properties and when it will not. The model argues that two grammatical features, segmentation a d hierarchical combination, appear in all settings in which one human communicates symbolically with another. These properties are preferentially assumed by speech whenever words are spoken, constraining the manual modality to a global form. However, when the manual modality must carry the full burden of communication, it is freed from the global form it assumes when integrated with speech--only tobe constrained by the task of symbolic ommunication to take on the grammatical properties ofsegmentation a d hierarchical combination. Perhaps the clearest example of the robustness of language comes from the fact that language is not tied to the mouth and ear but can also be processed by the hand and eye. Sign lan-guages of the deaf have been found to take over all of the func-tions and to assume the grammatical properties characteristic of spoken languages (e.g., Klima & Bellugi, 1979). Moreover, when exposed to a conventional sign language such as Ameri-can Sign Language, deaf children acquire the language as effort-lessly as hearing children acquire spoken language (Newport & Meier, 1985). Thus, the manual modality can serve as a me-dium for language, suggesting that the capacity for creating and learning a linguistic system is modality independent. However, communication i the manual modality does not always as-sume languagelike properties. In particular, when hearing adults and children use their hands to gesture as they speak, those gestures do not take on the grammatical properties char-acteristic of speech (McNeill, 1992). In this article, we propose a model that explains when th