Have you ever noticed that similar movies seem to come out around the same time? There are so many examples: Antz and A Bug’s Life both came out in 1998; 2006 featured The Prestige and The Illusionist, both with dreamy leading men playing peculiar magicians in the 1800s.

In 2016, both DC and Marvel’s good guys were fighting each other in Batman v Superman: Dawn of Justice and Captain America: Civil War.

Films with similar themes that are released around the same time are called twin movies.

This trend has been happening almost as long as movies have been on the big screen.

The earliest example involves one of the most successful movies of all time. Bob Mondello of NPR’s All Things Considered wrote about these pre-WWII twin movies:

“When Gone with the Wind was still auditioning Scarlett O’Haras, Warner Bros. decided to steal MGM’s thunder by beating it into theaters with a Confederacy epic of its own. It bought the rights to the Broadway play Jezebel, cast Bette Davis as its vain, self-destructive Southern belle, shot it more cheaply in black and white, and opened it while Gone with the Wind was still shooting. Jezebel won Davis an Oscar, which gave Warner Bros. bragging rights in February 1939—bragging rights that pretty much evaporated a few months later.”

Gone With the Wind would go on to win 10 Academy Awards, and adjusted for inflation, as of 2014, it was “still the most successful film in box-office history.”

Sometimes this phenomenon makes complete sense.

In 1992, for instance, two movies related to Christopher Columbus’ journey across the Atlantic were released, 1492: Conquest of Paradise and Christopher Columbus: The Discovery. As Mondello noted, the similar release date of these films is “entirely understandable, considering 500th anniversaries don’t come around often.”

Other times, certain scripts seem to serendipitously fall out of the air at the same time.

Douglas McGrath, screenplay author of Infamous, had one such experience when he called his friend Bingham Ray with some exciting news. The conversation was recalled in an interview with the New Yorker‘s John Seabrook:

“Good news,” McGrath remembered telling Ray in a phone call, “I finished my script!”

“I know,” Ray said. “I’ve got it on my desk!”

“And I paused,” McGrath recounted to Seabrook, “and I said, ‘Uh, no you don’t, because I have it on my desk.’”

“No, it’s right here,” Ray protested. “Capote, by Dan—” There followed, McGrath told the New Yorker writer, “what is known in the WASP community as a polite pause.”

Later in the interview, McGrath would muse, “What are the chances of two scripts about Truman Capote going out at the same time?”

There are also “mockbusters,” low-budget films that are released straight to DVD when mega blockbusters hit the big screen.

This Wikipedia entry describes the theory behind mockbusters: “A mockbuster may be similar enough in title, packaging, etc. in hopes that consumers confuse it with the actual film it mimics, but their producers maintain that they are simply offering additional products for consumers who want to watch additional films in the same subgenres.”

The real reason for twins might be simple competition.

Studios are constantly acquiring scripts as investments. When executives learn that a competing shop is working on a film with a similar premise to a script they own, it can become a race to put out the film first in order to get the most out of their investment in a script.

And 2017 already has some twins in the works. Look for The Disaster Artist and Best F(r)iends, both of which tell the story of the relationship between the creators of the cult movie The Room.