American elections are peculiar instruments of democracy, because they are so consistent in whom they leave out. In the past three Presidential elections, about forty-five per cent of those eligible to vote chose not to. And although this fact has been the subject of some public-spirited anxiety, it has generally not troubled political scientists too much, because it seemed as if non-voters had more or less the same view of the parties as voters did.
http://www.newyorker.com/news/benjamin-wallace-wells/sanders-trump-and-the-rise-of-the-non-voters
