If you can't explain it......................

When things go wrong, as in the crisis, the cause is not necessarily irrational behavior, nor an external shock, but possibly a mismatch between the chosen heuristic [aka "a rule of thumb"] and the environment...
     Two real-life examples from financial markets show what this abstract  description of decision-making means in practice.  The first concerns J. P. Morgan and its British-born banker Sir Dennis Weatherstone, who started as a bookkeeper at the age of sixteen and rose to become CEO in 1990.  The challenge was how to decide which of the many new and obscure financial products suggested by the traders and mathematicians on its staff the firm should sell to its clients.  With no past history for the performance of those products, there was no basis for judging which ones were likely to be effective.  The new products were an example of radical uncertainty.  The strategy Weatherstone employed was to make sure that any new product was understood by senior management.  The narrative underlying the strategy was that if the product could be explained in a conversation among senior managers then there was less risk that something might go badly wrong.  Weatherstone, I am told, would give the inventors three slots of fifteen minutes to explain the products to him.  If at the end of that he still did not understand the product, the firm would not sell it.  In 2008 there must have been many executives who wished they had followed Weaterstone's heuristic.

-Mervyn King,  The End of Alchemy:  Money, Banking, and the Future of the Global Economy