Conventional wisdom has long held that the resumption of large-scale American beer production after Prohibition led to the use of 'adjuncts' not typically found in European beer—most notably, corn, rather than a full grain bill of barley or a combination of barley and wheat. These cheaper ingredients led to a bland, watered-down brew, and until the resurgence of craft beer in the US over the past 20 years, it was impossible to find a beer made with ingredients that America's first European settlers would have recognized; at least, that's how the story goes. The truth is a bit more complicated, and it goes back to the Mayflower.
'beer myths' on Serious Eats
There are many myths swirling around the origins and evolution of porter and stout. First there is the notion that stout and porter refer to quite different styles; another holds that these beers were always dark, while a third tradition relies on the 'three threads' story to give porter an origin myth. All these tales are largely—and in some cases entirely—untrue.