This Harvard Business School study on the fear of speaking up at work should spark discussion in many workplaces, but it probably won't in the ones where it is most needed.
One of the hidden fears in many workplaces that I've seen: The concern that some form of retaliation will occur, even if it is years later. Often, that fear is triggered by various stories that are circulated, some of which may be completely inaccurate.
Managers need to be on the alert for those rumors and respond in a credible manner. If management botched a decision, the mistake should be admitted. If the story is wrong, then it should be corrected. Water cooler legends, however, should not be ignored. They can help to feed the fear.