It's the question that every movie fan asks in summer: why are there so many remakes and sequels and reboots? It turns out that science may have an answer. Unfortunately, if you're hoping for more original stories, the prognosis is not good.

Two network theorists in the Netherlands, Folgert Karsdorp and Antal van den Bosch, just published a study on story networks in Royal Society Open Science. Story networks, they write, are "streams of retellings in which retellers modify and adapt retellings in a gradual and accumulative way." There is also a basic structure that seems to underly how these networks function. To explore retellings, the researchers looked at more than 200 versions of the Little Red Riding Hood story, which had been retold over the past two centuries. They measured the stories' similarity to one another with the amusingly named "bag-of-words" technique, which reveals how many words two texts have in common. Then they created a network diagram showing relatedness between stories over time. Earlier stories became what the researchers called "pre-texts" that inspired later retellings.

Translated into movie terms, you can think of Bram Stoker's original Dracula novel from 1897 as a pre-text, and all of the subsequent movies and TV series as retellings. A story network grows out of Dracula as people retell the story, then retell the retellings, modifying it as they go. What the researchers found was that retellers rarely went back to the earliest pre-texts but instead preferred to retell more recent versions. In the case of the Dracula story, that would explain why a terrifying, barely human monster in the late nineteenth century is commonly represented today as an ultra-hot guy with sexual magnetism who occasionally goes fangy. As the story got retold throughout the twentieth century, you can see Dracula getting more and more handsome with each retelling, until we expect that Dracula is a suave and charming man with a tragic past. As retellers gravitated toward the most recent retelling, certain aspects of the story were magnified (such as Dracula's hotness) while others were forgotten (for example, we have yet to see a single Dracula retelling that deals with a forgotten aspect of the novel, which is that Dracula's love interest, Mina, is a geek who uses all the latest Victorian recording technology to do research on vampires).

That said, the researchers also found that a very small percentage of pre-texts spawned most of the retellings, which explains those big nodes you see in the network diagram. To go back to Dracula, that means some retellings, such as the Bela Lugosi movies of the 1930s, are vastly more influential than others. There will always be the really odd interpretations or attempts to go back to the source material, but in general these will not become highly connected nodes in a story network. As the researchers explain, "Some story versions are used only once to produce a retelling, whereas others serve as pre-textual context for many other stories and could be called ‘story hubs’." Still, the influence of any pre-text decays exponentially over time. That means new retellings will go back to earlier source material less and less often as time goes on.

The question is whether we can use this analysis to understand the explosion in sequels at the box office. Obviously a sequel is not the same thing as a retelling, but it would be hard to argue that the Star Trek franchise isn't a story network of some kind. The researchers suggest this could be an area for further research if we were to study stories as "small world networks." In that scenario, they write, "stories would be connected to only a few other stories, while at the same time all stories in the network would be connected to each other through only a few intermediate steps." Each Star Trek series might be a small world network, linked to each other. Or you could go bigger and consider the Star Trek franchise as a small world network that's linked to the networks of other space operas like Star Wars and Battlestar Galactica.

No matter how we analyze the networks of sequels, what's important is that we have evidence that story networks grow when people retell one or two recent stories over and over again. And that might help explain why summer movies tend to be a lot of sequels alongside a few original flicks. You can think of Star Trek, Avengers, or (gulp) Warcraft as those big nodes in the story network, while standalone original movies like Gravity or Wall-E are the exceptions that prove the rule. Some stories are destined to be retold again and again, whether as reboots or sequels. Others stand alone.

Royal Society Open Science, 2016. DOI: 10.1098/rsos.160071