One reason that should be examined for the middle class in the United States being "on the edge" is that "middle class" people of today believe they "require" more luxury goods than in generations past.  Many people in "middle class" are two paychecks from the poor house because they accumulate personal debt at unprecedented levels in order to purchase bigger homes than they can afford, additional cars that they really don't need, home electronic gadgets that didn't exist a few years ago, and clothing for kids that is far more expensive than any child should wear.  In past generations, when middle class people couldn't afford to pay for something, they either didn't buy it or saved up until they could make the purchase.  Many American "middle class" people live on the edge because they spend money as if they are "upper" middle class and desire the lifestyle of the wealthy.