I actually had the same theory, I was thinking that the increased valuation of the stocks caused the "growth" rather than earnings performance. I'm guessing a good way to test this theory would be to obtain a historic chart of average earnings increase %'s of stocks within the market and another...
Thats what I think is responsible for the economic boom of the 90s.
More people spent their money on [unnecessary] goods, a part of it due to the ease and availability of credit from banks and credit cards. More spending = higher sales and growth for many companies. This increased spending...