Philip Athans’s Reviews > Unutterable Horror: A History of Supernatural Fiction > Status Update

"I will maintain that the establishment of Weird Tales itself was the definitive beginning of “supernatural horror” as a distinct genre, just as the establishment of Amazing Stories in 1926 canonically introduced the genre of science fiction."

I agree--the various genres as we know them today were codified in the era of the American pulp magazines, about the first four decades of the 20th century.