Umm
But the consequence was that the US stock markets, which were still open at the time that the announcement was made, rose by approximately 9%, although they did not recover all the losses that Trump inflicted upon them despite doing so.
….
All these factors should have complicated and reduced market euphoria last night. It appears that they did not. For anyone who claims that markets are efficient and take all factors into account when pricing because they are possessed of a large amount of information about future actions, all of which they can accurately price, yesterday was the clearest indication that such an idea is complete and utter nonsense. Markets did no such thing. They took one piece of information and reacted to it without ever seeking to contextualise what might be happening.
The efficient markets hyupothesis says that generally known information is already in market prices. Stronger versions say that privately known information is already in market prices. Therefore it is new information that moves market prices.
EMH doesn’t say market prices are *right*. It says market prices invclude all the information folk have and what they think about it.
So, new information moves market prices is not a disproof of the EMH. It’s rather a proof of it….