From Edge.org:
Writing is inherently a topic in psychology. It's a way that one mind can cause ideas to happen in another mind. The medium by which we share complex ideas, namely language, has been studied intensively for more than half a century. And so if all that work is of any use it ought to be of use in crafting more stylish and transparent prose. From a scientific perspective, the starting point must be different from that of traditional manuals, which are lists of dos and don'ts that are presented mechanically and often followed robotically. Many writers have been the victims of inept copyeditors who follow guidelines from style manuals unthinkingly, never understanding their rationale. For example, everyone knows that scientists overuse the passive voice. It's one of the signatures of academese: “the experiment was performed” instead of “I performed the experiment.” But if you follow the guideline, “Change every passive sentence into an active sentence,” you don't improve the prose, because there's no way the passive construction could have survived in the English language for millennia if it hadn't served some purpose.
The problem with any given construction, like the passive voice, isn't that people use it, but that they use it too much or in the wrong circumstances. Active and passive sentences express the same underlying content (who did what to whom) while varying the topic, focus, and linear order of the participants, all of which have cognitive ramifications. The passive is a better construction than the active when the affected entity (the thing that has moved or changed) is the topic of the preceding discourse, and should therefore come early in the sentence to connect with what came before; when the affected entity is shorter or grammatically simpler than the agent of the action, so expressing it early relieves the reader's memory load; and when the agent is irrelevant to the story, and is best omitted altogether (which the passive, but not the active, allows you to do). To give good advice on how to write, you have to understand what the passive can accomplish, and therefore you should not blue-pencil every passive sentence into an active one (as one of my copyeditors once did).
More here.
