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What is passive voice?
In English, all sentences are in either "active" or "passive" voice:
active: Werner Heisenberg formulated the uncertainty principle in 1927.
passive: The uncertainty principle was formulated by Werner Heisenberg in 1927.
In an active sentence, the person or thing responsible for the action in the sentence comes first. In a passive sentence, the person or thing acted on comes first, and the actor is added at the end, introduced with the preposition "by." The passive form of the verb is signaled by a form of "to be": in the sentence above, "was formulated" is in passive voice while "formulated" is in active.
In a passive sentence, we often omit the actor completely:
The uncertainty principle was formulated in 1927.
When do I use passive voice?
In some sentences, passive voice can be perfectly acceptable. You might use it in the following cases:
The actor is unknown:
The cave paintings of Lascaux were made in the Upper Old Stone Age. [We don't know who made them.]
The actor is irrelevant:
An experimental solar power plant will be built in the Australian desert. [We are not interested in who is building it.]
You want to be vague about who is responsible:
Mistakes were made. [Common in bureaucratic writing!]
You are talking about a general truth:
Rules are made to be broken. [By whomever, whenever.]
You want to emphasize the person or thing acted on. For example, it may be your main topic:
Insulin was first discovered in 1921 by researchers at the University of Toronto. It is still the only treatment available for diabetes.
You are writing in a scientific genre that traditionally relies on passive voice. Passive voice is often preferred in lab reports and scientific research papers, most notably in the Materials and Methods section:
The sodium hydroxide was dissolved in water. This solution was then titrated with hydrochloric acid.
In these sentences you can count on your reader to know that you are the one who did the dissolving and the titrating. The passive voice places the emphasis on your experiment rather than on you.
Note: Over the past several years, there has been a movement within many science disciplines away from passive voice. Scientists often now prefer active voice in most parts of their published reports, even occasionally using the subject "we" in the Materials and Methods section. Check with your instructor or TA whether you can use the first person "I" or "we" in your lab reports to help avoid the passive.
To learn more about the use of passive voice in the sciences, visit our handout on writing in the sciences.
When should I avoid passive voice?
Passive sentences can get you into trouble in academic writing because they can be vague about who is responsible for the action:
Both Othello and Iago desire Desdemona. She is courted. [Who courts Desdemona? Othello? Iago? Both of them?]
Academic writing often focuses on differences between the ideas of different researchers, or between your own ideas and those of the researchers you are discussing. Too many passive sentences can create confusion:
Research has been done to discredit this theory. [Who did the research? You? Your professor? Another author?]
Some students use passive sentences to hide holes in their research:
The telephone was invented in the nineteenth century. [I couldn't find out who invented the telephone!]
Finally, passive sentences often sound wordy and indirect. They can make the reader work unnecessarily hard. And since they are usually longer than active sentences, passive sentences take up precious room in your paper:
Since the car was being driven by Michael at the time of the accident, the damages should be paid for by him.
Weeding out passive sentences
If you now use a lot of passive sentences, you may not be able to catch all of the problematic cases in your first draft. But you can still go back through your essay hunting specifically for passive sentences. At first, you may want to ask for help from a writing instructor. The grammar checker in your word processor can help spot passive sentences, though grammar checkers should always be used with extreme caution since they can easily mislead you. To spot passive sentences, look for a form of the verb to be in your sentence, with the actor either missing or introduced after the verb using the word "by":
Poland was invaded in 1939, thus initiating the Second World War.
Genetic information is encoded by DNA.
The possibility of cold fusion has been examined for many years.
Try turning each passive sentence you find into an active one. Start your new sentence with the actor. Sometimes you may find that need to do some extra research or thinking to figure out who the actor should be! You will likely find that your new sentence is stronger, shorter, and more precise:
Germany invaded Poland in 1939, thus initiating the Second World War.
DNA encodes genetic information.
Physicists have examined the possibility of cold fusion for many years.