The active voice refers to a sentence format that emphasizes the doer of an action. For example, in the sentence “The mice inhaled the tobacco-infused aerosol,” the doer, i.e., “the mice” seem important. On the other hand, in the passive voice, the action being performed is emphasized, and the doer may be omitted, e.g., “The tobacco-infused aerosol was inhaled (by the mice).” In this construction, “by the mice” can be omitted, which means that the reader already knows or does not need to know who the doer is; only the action is important.
The passive voice lends an impersonal tone, which is perceived to be formal, but can make the text more wordy and difficult to understand, especially when used in long sentences. Until recently, this tone was considered favorable for scientific writing and authors were advised to strictly avoid using the active voice, especially the use of “I” and “we” in their academic research papers. Compare “In this study, we investigated the effect of drug X on the serum levels of phosphorus under various conditions” with “In this study, the effect of drug X on the serum levels of phosphorus was investigated under various conditions.”
However, nowadays, many authorities are going against this traditional notion and encouraging the use of the active voice, with the view that academic papers should be easy to read and understand. In fact, multidisciplinary SCI-indexed journals like Nature and specialist journals like the American Journal of Botany, in their instructions for authors, state that the active voice is preferred. This is why you would now find the usage “In this study, we investigated…” very common.
So which should you use? The answer is a combination of both.
The active voice is especially useful in the introduction and discussion sections of your manuscript, where you discuss previous research and then introduce your own. For example, consider the following sentences: “Previous studies have established that drug X increases the serum levels of calcium in women with osteoporosis. In this study, we investigated the effects of drug X on the serum levels of phosphorus in post-menopausal women.” Here, using the active voice in the second sentence helps the reader make a clear mental transition from previous studies to the present study.
On the other hand, the passive voice is useful in the Methods section, where the steps taken are more important than the doer. For example, in a surgery document, a reader would prefer “A catheter was inserted for post-operative bladder irrigation” over “We inserted a catheter for post-operative bladder irrigation.”
Your guiding principle should be clarity: Think about what information the target reader is looking for, and choose the active voice or the passive voice, whichever will make the text most clear and comprehensible. If you write keeping this in mind, no journal reviewer will need to give you feedback about the active and passive voice.
Can you think of some other examples where either one voice—active or passive—clearly seems better?
For further reading, you could refer to this helpful article: Is it acceptable to use first person pronouns in scientific writing?