The economist has a good article on this with a number of hypotheses: <a href="http:&#x2F;&#x2F;www.economist.com&#x2F;blogs&#x2F;buttonwood&#x2F;2016&#x2F;02&#x2F;markets-and-economics" rel="nofollow">http:&#x2F;&#x2F;www.economist.com&#x2F;blogs&#x2F;buttonwood&#x2F;2016&#x2F;02&#x2F;markets-an...</a>