I'll start with active voice because it's simpler. In an active sentence, the subject
is doing the action. A straightforward example is the sentence "Steve
loves Amy." Steve is the subject, and he is doing the action: he loves
Amy, the object of the sentence.
Another example is the title of the Marvin Gaye song "I Heard It
through the Grapevine.” "I" is the subject, the one who is doing the
action. "I" is hearing "it," the object of the sentence.
What Is Passive Voice?
In passive voice, the target of the action gets promoted to the
subject position. Instead of saying, "Steve loves Amy," I would say,
"Amy is loved by Steve." The subject of the sentence becomes Amy, but
she isn't doing anything. Rather, she is just the recipient of Steve's
love. The focus of the sentence has changed from Steve to Amy.
If you wanted to make the title of the Marvin Gaye song passive, you
would say "It was heard by me through the grapevine,” not such a catchy
title anymore.
Why Is Passive Voice Recommended for Science Writing?
An exception is that scientists are often encouraged to write in
passive voice to lend their writing a sense of objectivity--to take
themselves and their actions and opinions out of the experimental
results. I used to be a scientist and I always found that odd. It felt
as if we were trying to hide that real people did the experiments.
Some scientific style guides
do allow for a limited use of active voice (1). For example, it may be
OK to write, "We sequenced the DNA," instead of "The DNA was sequenced,"
but it's still considered bad for scientists to insert themselves into
conclusions. For example, it would be bad scientific form to write "We
believe the mutation causes cancer." But you still don't need passive
voice to achieve your goals. For example, the active sentence "We
believe the mutation causes cancer," could be changed to "The data
suggests that the mutation causes cancer." That's still active, but it
eliminates the sense of subjectivity.
Is Passive Voice OK in Crime Reports?
On the other hand, sometimes passive voice does have advantages. For
example, if you truly don’t know who is taking the action, then you
can’t name the person. This is especially common with crime reports. For
example, a security guard might write "The store was robbed," because
nobody knows who the robber was.
IT'S NICE!