This leads to another main question relating to the success of the behavioural finance field. Some even predicted in the beginning of the 90s that the stock markets were overvalued and told investors to short sell. Nevertheless is behavioural finance not just another theory that tries to bring down the constitutional theory but it effectively helps to understand ex-post the behaviour of investors. This leads to explanations of the behaviour of bubbles and led academics like Fama or Malkiel, two defenders of the expected utility hypothesis, to accept the fact that sometimes markets are not fully efficient. So the
behavioural finance seems to explain very well why there is such a high trading activity, which undermines the assumption that prices only react to new information.