A theory or model of language development has to account for all languages,
whether spoken or signed. In this context, particular focus needs to be given
to deaf users of signed languages, given the unique situation of this group and
the varying circumstances under which they acquire language. Taking these
variables into account, we introduced a new approach to investigating deaf
children’s vocabulary knowledge in BSL by specifically measuring the degree
of strength of the mappings between form and meaning for items in the core
lexicon and associations between signs. The data we presented in this article
show that the idea of degrees of strength of the mapping between form and
meaning to measure language users’ vocabulary knowledge appears to hold
for signed languages as well as spoken. This is important for at least two
main reasons. First, it facilitates the comparison of vocabulary development
between deaf and hearing language learners. Second, given the overall lack of
suitable assessment tests for sign language and the overreliance of teachers of
the Deaf on tests that have been developed and normed on hearing language
users, our methodology shows that existing models of language development
can be successfully adapted across modality in a way that is appropriate for
deaf test-takers.