The models posited in Chapter 2 are applied to the problem set out in Chapter 1. Sources in Greek, Elamite, and Old Persian, taken together, give every reason to believe that mass nonnative acquisition of Persian was underway from the reign of Darius I onward. The cooperation of multiethnic groups of armed forces, work forces, and especially domestic staff, such as concubines and eunuchs, with their Persian-speaking masters evidently played a large role in the formation of Middle Persian. Contrary to a widespread assumption, Aramaic turns out not to be a lingua franca of the Achaemenian Empire, except at the level of provincial administration, but rather Persian was probably the best candidate for such a role. Middle Persian arose from Old Persian through a process of semicreolization as the term was defined carefully in Chapter 2.

Final reflections on the meaning of the transformation of ancient Persian, from Old to Middle Persian, put these events in the context of growing human mobility, migration, and population contact from the first millennium BCE until today, with its effects on language. This study has also unexpectedly shed light on the role of conquered people, particularly enslaved people in the domestic spaces of the Persians. Such people have left very little trace otherwise, but their role in the shaping of the Persian language and culture is remarkable. Their effects on Persian culture are still evident in the reduced morphology of Persian until toay. Prospects for new research linguistic history along these lines come into view.