If one assumes the EPP, it follows that clauses without overt subjects must have a null one. One question this assumption raises is what category can satisfy the EPP in a NSL. Related to this question is the status of overt subjects: are they in an A or A′ position? Do they have properties similar to dislocated phrases?

Several researchers have proposed a parametric version of the EPP. For example, Alexiadou and Anagnostopoulou (1998) argue that verb movement to I satisfies it in languages like Greek or Spanish that have rich agreeement. However, Holmberg and Nikkane (2002) point out that in Finnish, the verb also has rich inflection which licenses null subjects in many contexts and it moves to I, but overt expletives are still required in certain contexts. This leads them to propose two distinct requirements: one that can be satisfied by a verb moving to I, and another one that is active in some languages but not in others, that forces the Spec, IP to be filled.

Others like Manzini and Savoia (2002) propose that the EPP entails a D feature in IP. Languages like English lack a specialized element to realize D, hence a DP must surface. Consistent NSLs, on the other hand, have a weak D, so they need not realize it at PF. Finally, Northern Italian dialects, which have a subject clitic, have a strong D which surfaces as the clitic.