Is it really difficult to check empirically? It seems to be as simple as finding the number of investors and the amounts invested and checking that historicall

Is it really difficult to check empirically? It seems to be as simple as finding the number of investors and the amounts invested and checking that historically against the interest rate while controlling for things like population and GDP (or some other measure for disposable income available to be invested) I guess it might be difficult to get numbers on how many people are playing the stock market and by how much, but you might be able to get that data from individual brokerages or looking at the number of brokerages and hedge-funds and checking their publicly filed financial reports.