As a research field, linguistic typology aims to uncover the range of structures and constructions that are possible in human languages for expressing core grammatical -- and by extension, cognitive and cultural -- concepts. Moreover, typological classifications reveal areal and genetic influences on language structure and shed light on processes of grammaticalization (cf. Croft 2002; Heine, Claudi, and Hünnemeyer 1991; Hopper and Traugott 1993).
Typological studies have long been the province of spoken language investigation, but it is important to extend the scope of research to sign languages. It is only by looking at language in both modalities -- that is, both spoken and signed -- that we can come to understand the full range of possibilities of the structure of human language.
Furthermore, only by extending typological research to signed language can we come to understand the influence of modality on language structure. Typological investigation of sign languages will bring us closer to answering currently debated questions about modality-driven similarities and differences between signed and spoken languages, the influence of the age of sign languages on their structure, and the influence on structure of the heightened potential for iconic and indexic representation in the visual-spatial modality (cf. Aronoff et al. 2003; Meier 2002).
Aronoff, M., Meir, I., Padden, C., and Sandler, W. (2003). Classifier constructions and morphology in two sign languages. In Emmorey, K. (ed.), Perspectives on Classifier Constructions in Sign Languages. Mahwah, NJ: Lawrence Erlbaum Associates, pp. 53-84.
Croft, W. (2002). Typology and Universals. Second edition. Cambridge: Cambridge University Press.
Heine, B., Claudi, U., and Hünnemeyer, F. (1991). Grammaticalization: a conceptual framework. Chicago: University of Chicago Press.
Hopper, P.J. and Traugott, E.C. (1993). Grammaticalization. Cambridge: Cambridge University Press.
Meier, R.P. (2002). Why different, why the same? Explaining effects and non-effects of modality upon linguistic structure in sign and speech. In Meier, R.P., Cormier, K., and Quinto-Pozos, D. (eds.), Modality and Structure in Signed and Spoken Languages. Cambridge: Cambridge University Press, pp. 1-26.