I just found this interesting article on Slate. I’ve pasted the first 4 paragraphs. You can find the full text here.

Modern Horror Defined By Edgy Realism Of The 1970s
by NPR  8, 2011. By the late 1960s, classic horror movies pioneered by Vincent Price and Boris Karloff had run out of steam. What took their place in the period after that was something different, edgier and altogether more terrifying.
“To some extent you could say that modern horror started with the Universal classics, but I do think there is this significant turning point starting in 1968,” says Jason Zinoman, author of the new book Shock Value: How a Few Eccentric Outsiders Gave Us Nightmares, Conquered Hollywood, and Invented Modern Horror. Zinoman tells weekends on All Things Considered guest host Robert Smith the horror film genre is bigger today than it has ever been, and many of the tropes and motifs we see in modern horror films come from the period of the late 1960s through the end of the ’70s.
“I think it was a huge transition from the old horror – (Boris) Karloff, Vincent Price and the old monster movies – to the kind of new brand of scares that you saw in Rosemary’s Baby, Alien and Halloween,” Zinoman says.