An important factor is that, in most sign languages studied to date, verbs can be modified to indicate their subject/object; that is, arguments can be inferred from the form of the verb – and this is an important condition for pro-drop. In contrast, in a surrounding spoken language, this need not be the case. For that reason, pro-drop is, for instance, possible in ASL and NGT but impossible in English and Dutch.

Note that the answer to this question also relates to information provided in Chapter 1. Crucially, a sign language has its own grammar and is not dependent on the spoken language of the environment. The sign language may therefore allow pro-drop, no matter whether the surrounding spoken language allows it or not.