The flood of documentary films which fills both classrooms and many hours of cable television can powerfully inform or misinform millions of viewers. We watch them regularly, and there is a psychological temptation to believe or trust what they show or say. But how reliable are they?
Documentaries can misinform in several ways. First, the images themselves can be misleading. Often, if no still or movie picture of a historical event is available, some "reenactment" or "simulation" is often shown. But any such footage is, at best, an educated guess, and not as reliable as actual historical photographs. Worse, when a reconstruction or simulation is too expensive to manufacture, stock footage from Hollywood films is often inserted. Hollywood is fine for entertainment, but lousy for informing and educating.
Even when actual footage or still photos are available, there is a double bias: first, of the original photographers on the scene, and then of the selections made for the film.
A second danger of documentary films is not in what you see, but in what you hear. Typically, several experts or eyewitnesses are interviewed on camera. Of the many hours spent interviewing, only a few minutes will wind up on camera - and those are often chosen, not for the information, but rather for the drama, which they present. And of the "experts" interviewed, it is understood that one who has a radical or iconoclastic interpretation to offer will be the most interesting on-screen, even if that alleged specialist is sadly mistaken. Historians and scientists whose views are outrageous rather than rational make for entertaining films, but not not for informative ones.
Even the background music can be misleading: a recent documentary about English history showed scenes of London in 1965, while playing "Won't Get Fooled Again," a song not recorded until 1971. The film gave the misleading impression that the song was written in mood of that historical moment, when it was in fact written at a time far removed.
A final strike against documentary films is that the actual amount of information - of quantifiable data - is rather small for the time investment. For sixty minutes spent watching a documentary, compared to sixty minutes spent reading a textbook, fewer facts are gained. Documentaries are simply an inefficient way to inform one's self about a topic.