It is sometimes argued that the sort of evolution that we have observed in
macroeconomic theory is not paralleled by the evolution of financial economics,
simply because the relation between theory and practice was much
closer in the latter. This would suggest that, since the theory was continually
faced with the acid test of empirical verification, it must have evolved
to become more consistent with the empirical evidence. It should become
clear, in what follows, that this is not the case. The efficient markets
hypothesis, which is at the heart of modern financial economics, says
that all the pertinent information concerning financial assets is contained
in the prices of those assets. The fundamental idea is that if there were any
information other than that contained in prices, some individual could
profit from it by an arbitrage. In other words, if someone held information
which meant that the change in the price of some asset was predictable he
would make money by using that information. Thus the conclusion is that
prices must follow an unpredictable path, or what is known as a ‘random
walk’. This idea was already developed by Bachelier (1900) in his thesis.
There is a certain ambiguity here which was present in the original work.
Prices contain all the available information because this is incorporated in
the demand and supply of those who receive the information. Thus, prices
reflect that information. But as Grossman and Stiglitz (1980) argued, if all
the information is contained in the price of an asset, nobody would have
an incentive to look at their own information and therefore nobody would
act on it. As a result the information would never be transmitted into the
price. Leaving this problem to one side, what Bachelier argued was that
individuals in the market independently observed independently distributed
shocks to the system, and this is what constituted their ‘information’.
His argument was that one could think of the small shocks as random and that the sum of such shocks would be normally, or Gaussian, distributed
with mean zero. Thus the sum of many small shocks observed independently
by the agents would lead to prices following a random walk. What
is not clear in his analysis is how, and when, prices are being set for this
to work.