Of the box office top 10 movies released in 2011, 9 were sequels, and the other was based off a TV show (Smurfs). Even within the sequels, 4 were ultimately spun off from TV shows (the Mission: Impossible and Transformers movies), or book series (the Harry Potter and Twilight movies). The Transformers movie also was based on a kids' toy line.
Can nobody think of any new ideas? It seems like everything is a sequel, prequel, remake, reboot, re-imagining, or a spin-off from an existing series in some other medium -- TV show, toy line, novel, etc. I'm still waiting for them to mine other domains in pop culture, like Peanut Butter Cups vs. Gushers: Annihilation.
Rewind only as far back as 1984, and just 2 movies in the top 10 were sequels: Star Trek III and Indiana Jones and the Temple of Doom. Only the Star Trek one was ultimately based on a non-cinematic source (a TV show). Ghostbusters, Beverly Hills Cop, Romancing the Stone, Gremlins, Footloose, etc. -- all new ideas. Scroll through Wikipedia's list of other releases at "1984 in film" and notice how few adaptations and sequels there are.
I wonder how far back this originality goes, though. With a lot of time, I might actually try quantifying this, but not now. The basic approach can be illustrated with one example year, say 1955, given that in many respects we're in a neo-'50s zeitgeist right now.
I think you'd give less weight to adaptations of literary material, since a book doesn't tell you much about how the movie should look, how the players should interact, what the sound should sound like, and so on. Remakes or sequels of movies would be weighted pretty heavily. And so would adaptations of plays. Those all provide a much more filled-in template at the outset of making the movie.
Of the top 10 movies of 1955, only 3 were fully original: Lady and the Tramp, Rebel Without a Cause, and Love Me or Leave Me (although it was a bio-pic). Movies adapted from novels accounted for another 2: East of Eden and The Sea Chase. The remaining half of the big hits were adapted plays: Mister Roberts, Guys and Dolls, The Seven Year Itch, Picnic, and Oklahoma!
That's nowhere as bad as all the lame adaptations we have today, but it's still striking compared to the originality of material from the '70s and '80s, probably starting out even in the '60s. I don't have a clue how original the material was in the '20s, and don't have the time to check systematically right now. But this is the kind of thing you can easily quantify and plot over time.
Most people who complain (rightly) about the sorry state of movies today do not realize that we're probably repeating the mid-century history of Hollywood. Back then it was radio and TV that were blamed for stealing away theater-goers, and now it's the internet and video games. In reality people in both periods wanted to stay at home all day, and chose radio and TV, or the internet and video games, over movies for that reason. They weren't helpless victims of a new technology, which they could always have chosen not to adopt -- just as most of us in the '80s didn't spend hours and hours every day playing video games, even though the option was there.
They tried all sorts of gimmicks to get people out of their houses, to give them an experience that they couldn't get with radio or TV. Widening the aspect ratio to look more panoramic (similar perhaps to the push for IMAX these days), the use of 3-D glasses, etc.
Maybe they thought the same thing about spinning off so many plays -- like, "Hey idiots, here's something you can't see on your idiot box." It was like visiting far-off Broadway, right in your own neck of the woods. Top-quality actors who could sing -- that probably wasn't available on early TV.
I wonder if they also felt like they'd hit a creative slump and figured, hell, it looks like the playwrights are making something that interests the general public. I mean, talk about widespread success -- a boring blowhard like Arthur Miller managed to marry Marilyn Monroe in 1956. Might as well copy whatever they're putting out, willy-nilly if we have to.
We tend to only remember the cool stuff from the past, like film noir from the mid-century, but often it was not very popular at the time. Same goes for It's a Wonderful Life, which flopped on release in 1946 and only found success in the 1970s and '80s. (I don't think most younger people or even 30-somethings tune in regularly these days when it airs before Christmas). This gives us a biased view of the past, and prevents us from seeing how similar it may have been to the present. And of course other periods in the past really were different from ours (like the Jazz Age vs. today).
Anyway, there's lots of room here for a good quantitative history of originality in source material for at least one domain in the art world.