Know Your Place
If we were to measure the effect of artwork based on how it speaks to the viewer, one would have to account for the world of the viewer.
Great art transplants the viewer into the world of the artwork. A viewer doesn't have to have experienced post-war elation or depression to take away a message or feeling. It is a juxtaposition of the collective experiences of the viewer combined with what the viewer is taking from the artwork.
If you are the art you are affected by, perhaps the ideal viewer is one that knows their place in the world.