You can’t beat the market. The efficient market hypothesis says that the price of a financial asset reflects all the information available and responds only to unexpected news. Thus prices can be regarded as optimal estimates of true investment value at all times. It is impossible for investors to predict whether the price will move up or down (future price movements are likely to follow a random walk), so on average an investor is unlikely to beat the market. This belief underpins ¬arbitrage pricing theory, the capital asset pricing model and concepts such as beta. The hypothesis had few critics among financial economists during the 1960s and 1970s, but it has come under increasing attack since then. The fact that financial prices were far more volatile than appeared to be justified by new information, and that financial bubbles sometimes formed, led economists to question the theory. Behavioral economics has challenged one of the main sources of market efficiency, the idea that all investors are fully rational homo economicus. Some economists have noted the fact that information gathering is a costly process, so it is unlikely that all available information will be reflected in prices. Others have pointed to the fact that arbitrage can become more costly, and thus less likely, the further away from fundamentals prices move. The efficient market hypothesis is now one of the most controversial and well-studied propositions in economics, although no consensus has been reached on which markets, if any, are efficient. However, even if the ideal does not exist, the efficient market hypothesis is useful in judging the relative efficiency of one market compared with another.