Damn good question.
How is middle class defined?
I recall learning some time ago that one indicator of true middle class status was having an income based on other people's work, such as a merchant or business owner. The test in this case was whether or not your personal income was dependent on the amount of work you perform personally. If you could stop working but your income remained stable, then you are middle class. The other test was to be a professional with excessive income, so that only a small portion of earnings are needed to maintain a middle class life style and the extra income was used to purchase income producing assets. This puts a professional into the middle class early in a successful career.
People whose income comes from assets purchased for them by others are members of the economic upper class when the income from the assets is high. These people do not ever have to acquire a profession or build a business in order to have a secure income. They have sufficient income that is not derived from their own work (professionals) or from the work of others whom they control (owners), This is, of course, inherited wealth.
The real point of this explanation, as I recall, was that most of America's self-described middle class is actually working class. If the work goes away, the income goes away, too. This was supposed to be the truest definition of working class and most people fit this definition, regardless of actual income levels. If you earn 100K a year from your job and need 100K just to keep your head above water, you have to work. You are then a member of the working class, even with a good wage.
I remember a lot of resistance to this idea when I first heard it discussed. I'll bet most debt-ridden, savings-light, work-a-day Americans who are pulling down 60K consider themselves middle class, too. But, lots of them are just eight paychecks away from poverty.
Keith