The phenomenon of twin babies appearing to communicate in their own “secret language” has long captured the imagination of both parents and researchers. Though many viral videos suggest a unique bond among twins—especially identical ones—experts clarify that true cases of a private language, known as idioglossia or cryptophasia, are actually quite rare and typically emerge only in extreme isolation. What’s more common is twins mimicking each other’s babbles, which is a natural and healthy part of early language development as their brains form speech connections.

One of the most beloved examples of this is a 2011 viral video featuring fraternal twins Merle and Stijn from the Netherlands. In the clip, Merle, dressed in pink, and her brother Stijn appear to be locked in a lively “conversation,” exchanging expressive babbles and intense eye contact. Their father, Roy Johannik, captured the heart-melting moment, which resonated with viewers around the world. Merle’s wide smile in response to her brother’s sounds only added to the charm, helping the video garner over 22 million views by early 2021.

Although the interaction between Merle and Stijn may look like a mysterious language exchange, research shows that about 40% of twins, particularly identical ones, may develop what is called an “autonomous language.” This doesn’t involve creating a new language but includes shared nicknames, sounds, or gestures that only they use with each other. These unique communication methods reflect the deep bond between twins and their tendency to interact closely, often developing exclusive ways to express themselves.

At the same time, studies indicate that twins may experience slight delays in speech and language development compared to singletons. This can be attributed to a combination of factors: less individualized interaction from parents juggling multiple children, and the twins’ tendency to mimic each other’s incorrect speech patterns. Because they often understand each other intuitively through non-verbal cues and shared behavior, they may have less immediate need for clear, conventional language. Fortunately, most twins typically catch up with their peers by school age.

In the end, while science doesn’t support the idea of a fully developed secret language between twins, moments like Merle and Stijn’s first “conversation” reveal a heartfelt emotional connection that words can’t always explain. Their sweet back-and-forth babbling and radiant expressions symbolize the special closeness that twins often share—an unspoken understanding that continues to fascinate and delight people everywhere.