Anyone watching the stock market has seen this: a post hits Twitter containing old news, and investors react as if it were new.

A working paper by INET's Ansgar Walther and Oxford University colleagues, Peiran Jiao and Andre Veiga, suggests the phenomenon is prevalent, creating a source of excess volatility for investors. It found evidence that the unmediated flood of news and opinion that pours over the social media transom results in some stocks getting whipped around for weeks by the same facts.

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