Passivisation and a North-South divide

If we go all the way back to Old English, we find that the Anglo-Saxons had passive constructions which could be used to demote a non-specific agent, much as we do now.

They also had another way to demote the subject, the so-called impersonal construction. These clauses had a (now extinct) impersonal pronoun subject man, which meant essentially one. Even using impersonal one as a subject is now obsolete, or at least extremely archaic, so in translations from Old to Modern English, most translators now turn these clauses into passives ("then Martinus commanded that they should be caught").

To investigate how the use of passivisation in English has changed over time, the authors compared parallel translations of the Rule of St Benedict in Southern and Northern dialects of Middle English with one in Old English. Comparing parallel texts allows for genre to be controlled, which is essential as the use of passive constructions is sensitive to genre. The Rule is a text of instructions, wherein the agent is often non-specific, providing ample opportunity for passivisation or any alternative ways of demoting the agent. The hypothesis is that a dialect which has alternative means to demote the agent will use the passive less.

The Anglo-Saxons had two ways of demoting an agent in Old English: the passive and the impersonal man construction. Sometimes they used one method to accomplish this goal and sometimes the other. Over time the impersonal pronoun man was lost, first in the South of England, then in the North. As a consequence, these particular impersonal constructions were lost as well.

Where people used to be able to choose between a passive and an impersonal man construction to demote a non-specific agent, they now have to use the passive. Assuming people don’t change the frequency at which they want to refer to non-specific agents (and there’s no reason to think they would), the frequency of the passive rises in consequence, since it’s now doing double duty.