After some off-the-cuff thinking about spin-off movies from today and the mid-century, I decided to look into the data in greater quantitative depth. To re-cap, the goal is to see whether there was another period like today when so many movies were either adaptations of an existing work or a sequel to an earlier movie.
Based on a hunch, it looked like the '50s were another such period. The strongest influence on the zeitgeist is the trend in the violence rate, so not even knowing very much about all the popular movies of that period, let alone before or just after, that was a good guess -- today and the '50s were both well into a falling-crime time. And based on a quick look at the '80s, a rising-crime period, it looked like there weren't so many sequels and adaptations.
Granted, movies are more than just narratives, and this approach totally leaves aside the question of visual originality. But it's a feasible way to take a crack at the central question. Also bear in mind that originality and greatness aren't the same thing. I'm not judging how great or enjoyable all these movies are because I haven't seen most of them. The focus is entirely on how original the process was that gave us the plot, characters, themes, verbal tone, and so on.
I went through every year from 1936 to 2011 and tallied how many movies in the box office top 10 were a sequel or adaptation. I didn't go back any further because there aren't easily accessible data on the top 10. Wikipedia only lists 5 to 7 for many years, and doesn't always have a link to information about the source of the story that far back. This will do pretty well, though, because that still gives us just about the entire falling-crime period of 1934-1958, the whole rising-crime period through 1992, and the whole falling-crime period since then.
Rather than stall the presentation and discussion of the data, I've put the methodology part at the very end, in case you want to know exactly how I nitpicked what did and did not count as an adaptation and a sequel. They weren't mutually exclusive, though: for example, I coded the new Twilight movie as both a sequel and an adaptation.
The first graph below shows the total number of movies with less original narratives (whether a sequel only, adaptation only, or both). Then just to probe whether the type of unoriginality matters, the second graph splits them apart to show how many were adaptations (regardless of whether or not they were also sequels), and how many were sequels (regardless of their status as adaptations).
Score another one for my theory. There's visible year-to-year variation, micro-periods of a few years that go against a trend, etc., but there are three clear movements -- up, down, and up. The first one climbs toward a peak around 1956-'58, then it starts to fall off toward an end-point around 1988-'91, and then rises again through the present. That just about perfectly tracks the trend in the homicide rate through three different movements up, down, and up again. So it's not a coincidence.
Also, it looks like most of this link to trend in the violence rate is due to how common adaptations are. Sequels show a more steady one-way growth over the entire period, and usually aren't as common as adaptations anyway. That makes the case even stronger -- at least if it were mostly due to sequels, you could say they're still pretty original, just carrying over the characters, themes, and maybe settings from one movie to another. Instead it's more due to borrowing all that stuff, plus a good amount of the plot, dialogue (or at least its verbal style), and even songs if it's a musical.
What are some potentially surprising things to notice in the main graph?
First, today's culture where so much is adapted or a sequel milking a popular series is not that old. Even as recently as the 1980s, movie narratives were a lot more original.
Second, contra the standard story in the world of film geeks, the '80s were a high point for originality -- more so than the New Hollywood period of the later '60s and early '70s, few of whose movies brought in many viewers. It was still a somewhat out-there thing to be making original movies in the '60s, and they struggled to find an audience. By the '80s, everybody was in the mood for exciting new stories, so there was no incompatibility between being mainstream and being original. The '60s were just the beginning; the culmination arrived in the '80s.
(That pattern shows up in popular music, too -- in the '60s the Velvet Underground was, well, underground, but David Bowie and Peter Gabriel topped the charts in the '80s. And the Velvets were just pointing the way back then.)
Third, it is true that the '50s were a period a lower creativity -- not zero, but just compared to the reversal that would begin in the '60s and peak in the '80s.
Fourth, even more importantly, the world was not static before 1960. Movie narratives weren't that unoriginal in the mid-'30s -- they gradually moved from there toward the peak of adaptations in the late '50s. Nobody gets more confused about history than when they think about the '50s and '60s. Yes, the '60s were a break from the '50s, but the conditions of the '50s didn't stretch back forever, or even several decades. The '20s were just about the opposite of the '50s; they pre-figured the '80s. These changes in the zeitgeist are more like cycles than a one-time disruption of a formerly static equilibrium.
(Similarly, popular music from the mid-'30s wasn't that bland -- it had only begun to come down from the height of the Jazz Age, when the focus was on fun, danceable melodies. It took decades of gradual erosion to reach the chart-topping hit "How Much Is That Doggie In The Window?" Rock 'n' roll was still off in the distance during the mid-'50s. And in architecture, the mid-'30s had only begun to wipe off the ornamentation and boil the elegance out of Art Deco. The Streamline Moderne of the later '30s and early '40s was just the first step toward the soulless mid-century minimalism.)
Fifth, economics tells us little or nothing about long-term changes in the creative part of movie-making. When we look at the first graph, we don't see the business cycle, or whatever, staring back at us.
It's too bad I can't apply these methods back to the rising-crime period of ca. 1890 or 1900 through 1933. I bet you'd see something like the 1960-1990 period, an initially higher level of adaptations that fell off to a bottom sometime in the '20s or early '30s. Yeah, I know they adapted Frankenstein and All Quiet on the Western Front, but lots of hits were not adaptations of specific works but of common legends (Intolerance, Robin Hood, The Thief of Bagdad). Not to mention all of the blockbuster classics that had original narratives, like Metropolis and King Kong.
Finally, let me emphasize yet again that this is only about how original the narratives are. Obviously a lot else contributes to overall originality, like the equally important visual elements. And not all adaptations have the same snore-inducing effect on a normal person. At least those adaptations from the '50s weren't based on children's cartoons and toy lines, although each year might have had several campy-looking musicals back then, so even they weren't as mature as we tend to think.
Data on the box office top 10 come from the various Years in film pages at Wikipedia. I then read through the Wikipedia entry on each movie to see what the source material was. Given how popular these movies are, and how easy it is for the editors to find out if a hit movie was based on a hit play or novel, this information wasn't lacking.
We all know what a sequel and prequel are, but just to clarify, I included both plot-based sequels, where the narratives are inter-related across the movies (like The Empire Strikes Back), and sequels based on the same character, tone, and themes, but with little or no continuity in plot (like most of the James Bond movies).
An adaptation is a different way of being less original in the storytelling, so I coded it separately. Just about the entire literary side of a movie can be grafted in from a novel, play, previous movie, TV series, etc. If the Wikipedia entry said it was only loosely based on the source, that the plot was extensively altered, etc., I didn't count it as an adaptation. In those cases, maybe they were trying to ride on the coat-tails of the source's success, but at least they made up an original story.
I counted anything that seemed long enough in form to be used in a movie -- not a single short story, not a news or magazine article, and so on. Serial stories, series of shorter children's books, etc., were counted. Biographies, autobiographies, memoirs, non-fiction books, and the like I judged case by case. If the real events described, and the subjective tone relating to them, were well known and almost legendary, I didn't count it as an adaptation of another's original work. If they were more stylized, bringing less known events to light, etc., I gave the original one credit for creating something, and counted the movie as an adaptation.
After awhile, even original works take on a legendary status, so that a modern movie-maker could still give their take on it, rather than borrow the plot entirely. There's no clear way to decide what's legendary and what isn't, so I just considered any work older than roughly 100 years at the time of the movie to be a legend, myth, fable, tale, etc., whether we know exactly who originated it (Romeo and Juliet) or not (Aladdin). If the movie was based on a newer work, which in turn was based on an older legendary work, I counted that as an adaptation, like the 1959 movie Ben-Hur that is based on a novel from 1880, although relating stories from the Bible.
Overall, though, these somewhat puzzling cases were not common in any single year: most adaptations are from earlier movies, stage plays, novels, and other long forms of fiction. So, quibbling wouldn't alter the 76-year pattern in the graphs.
Lastly, if a movie was a sequel to an adaptation, I counted it as both. So the first Star Trek movie was just an adaptation, and the rest after it were both.