Despite the enormous attention that pro drop has received in the linguistic literature, there is no generally accepted answer to the question why relatively rich Germanic languages do not have argumental null subjects, neither is there a fundamental answer to the question why English would not allow them in at least 3SG contexts, where the agreement marker uniquely identifies the features of the unexpressed subject, just like in Italian. We argue that a closer inspection of the Germanic languages reveals that tense and agreement are expressed mono-morphemically, whereas Romance pro drop languages have distinct morphemes for tense and agreement. This allows us to postulate that the lack of pro drop in Germanic languages is a consequence of overspecification: the presence of the tense features makes licensing of a null subject impossible. Germanic variants that have partial pro drop, such as Frisian and Bavarian German, can be naturally accommodated in our approach by reference to complementizer agreement.
Koeneman, Olaf and Zeijlstra, Hedde
"The Lack of Full Pro Drop as a Consequence of Featural Overspecification,"
University of Pennsylvania Working Papers in Linguistics: Vol. 28:
1, Article 10.
Available at: https://repository.upenn.edu/pwpl/vol28/iss1/10