Twins are notable for how much they share. After sharing the womb, twins often have a deep emotional connection, and, in the case of identical twins, they share most of their DNA.

But the twin bond doesn’t end there. Those born simultaneously (both identical and fraternal twins) also seem more likely to develop a private language in early childhood that only they can understand.

It’s called cryptophasia, rooted in the terms "crypto," meaning secret, and "phasia," meaning speech. Researchers and parents have been trying to make sense of it for years. Is it a real language or a double dose of gibberish?

Talking the talk — and walking the walk

Twin speak is also sometimes called idioglossia, a similar phenomenon, but one that doesn’t include the mirrored actions and mannerisms that typically accompany cryptophasia. That is, twins don’t just speak in sync, they often move in sync, too.

What’s fascinating is that up to 50 percent of twins may engage in this kind of secret speech. For many people, this is proof that the special bond between twins imparts some sort of superhuman communication power. As Becca Jarzynski, a pediatric speech-language pathologist and clinical instructor at the University of Wisconsin-Eau Clare, writes on the American Speech-Language-Hearing Association blog:

The theory behind this “twin language” goes a little something like this: twins are so close to each other and rely on each other so much that they don’t have as much of a need to communicate with the outside world, so they make up their own idiosyncratic language that develops only between the two of them. It’s a fun and almost magical idea, for sure. But does it stand up to reality?

Well, yes and no. Examples do exist of twins who created what appears to be a well-developed secret language. In most cases, these twins were raised in extreme isolation and lacked adequate interaction with caretakers. One of the most famous examples is June and Jennifer Gibbons, identical Welch twins who only spoke to each other and used a strange language no one else could decipher. The duo turned to crime when they were 18 and were profiled in a 1986 book called “The Silent Twins.”

Then there's Poto and Cabengo. That's what the identical twins Grace and Virginia Kennedy called themselves. The Kennedys also spoke in what seemed to be a highly developed dialect of their own making until they were 8. They were featured in a documentary by Jean-Pierre Gorin, released in 1980.

Not so fast

But a lot of research suggests that most twin talk isn’t typically a fleshed-out language. Instead, it may have more to do with language delays (specifically, problems putting speech sounds together), which seem to affect twins in greater numbers than singletons. The reasons for these delays are complicated — likely related to the fact that twins are more often born prematurely, have lower birth weights and spend less time interacting (and speaking ) one-on-one with their parents and others. New research also suggests there could be genetic factors at play.

According to this research, twins share a close bond, but what that bond actually does is reinforce and prolong these lags in speech. Writes Jarzynski: “While this does make it kind of a ‘twin language’ (because the two twins seem to understand each other when others cannot), it’s also a delay in speech sound development that might need to be addressed by speech therapy.”

In other words, twins may spend a longer period of time than non-twins mirroring each other’s baby talk and speech mistakes. Their babbling can sound like a cryptic vernacular because they’re often on the cusp of real speech and wield all the conversational rhythms, intonations and structure of real conversations. But most twin speak lacks the complexity, extensive vocabulary and structure of a true language.

Cryptophasia usually declines as real speech kicks in, further supporting the idea that it’s typically just a temporary communication step as twins learn to talk for real.

Whatever you believe about twin speak, it’s fascinating to hear and watch. Check out these videos and judge for yourself.

Diaper-clad twin boys have a hilarious conversation:

Twin girls explain a card game to each other:

Phone conversation between 32-month-old twins:

Grown twins Matthew and Michael Youlden discuss the many languages they speak, including a secret twin language from their childhood that they’re expanding into a fully functional language: