The horror film genre evolves from the German silent classics of the 1920s to America's Universal monster movies of the 1930s, like Dracula and Frankenstein, before declining in the 1940s.
Post-WW2 fear of apocalyptic nuclear destruction stirs a new era of horror films in the 1950s, based around the science fiction of atomic mutations and alien invasions. Then Alfred Hitchcock's Psycho changed it all.
Horror films reflect the frightening, rapidly changing times of the 1960s and 1970s, epitomized by George A. Romero's Night of the Living Dead and continued by Rosemary's Baby, The Exorcist and Texas Chain Saw Massacre.
In the 1980's, 24hr cable news was dominated by the threat of new modern terrors. Moviegoers embraced iconic slashers like Freddy Krueger and Jason Voorhees; vampire films saw a resurgence.
The emergence of cellphones and new technology in the 2000's exposes Americans to new and perpetual terrors; horror filmmakers adapt, and seminal films like "The Blair Witch Project" open new doors; Blumhouse reinvents the genre with new nightmares.