Passivisation and a North-South divide
The study in a sentence
In contemporary English, one function of the passive is to demote the subject or agent of the sentence (e.g. to divert attention away from it). This study shows that availability of an alternative construction for demoting the agent in Middle English (the impersonal pronoun man 'one') decreases the need to use the passive, making it less common.
The study shows this by comparing translations of the same 15th century instructional text into Northern and Southern dialects of Middle English, which lost the man pronoun at different times.
“Mistakes were made” is a classic strategy to avoid responsibility for one's actions. It works because passive clauses allow you to take the agent (performer) out of subject position and omit it altogether.
There are less nefarious reasons for using the passive. Consider the following passive sentences:
Ice cream isn’t eaten in the Arctic.
My house was built in 1900.
The bread must be baked for 30 minutes.
Who is the agent in each case? Try adding the agent back in, using a by-phrase. Do they have anything in common?
In each case, the sentence works as agentless passive because the agent can be anyone or just someone: it does not refer to a specific person, unknown or not important. It makes sense to demote the agent and put a more important element at the beginning of the sentence (here: ice cream, my house, the bread).
English has always had a passive, but it is used more frequently now than it used to be. Why is that the case?
Within the constraints of their grammar, speakers generally try to make things as easy as possible for their listeners to process. This can be done by ordering the information they are providing in a way that is helpful to them, also known as information packaging.
When a language stably has different ways of saying “the same thing”, such as how the active and passive have existed side by side for thousands of years, different information packaging properties of the constructions emerge. Otherwise the forms are competing and one will eventually be lost (see the page on indefinite pronouns).
The information packaging of the passive has two roles: to demote the agent (to the point of not even having to mention it), and to promote the theme. The passive is therefore useful when the agent is not important (or you just don’t want to mention it - useful for politicians when “mistakes were made”) and/or when you want to focus on the theme argument by placing it first.
What can the rise of the passive tell us about the evolution of English?
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").
Rule of St Benedict
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.