Idioglossia (autonomous language) occurs in a percentage of twins prior to learning conventional language due to their continuous close proximity.
"And they developed idioglossia, which happens with—you probably know this—a certain percentage of twins: a language before they learn to speak English. They evolve because they're so close to one another all the time, and it's a guttural, syllabic uh form of communication." (said at 1:39:30)
Idioglossia (also termed autonomous language or cryptophasia) is a well-documented phenomenon in twin literature. Studies show it occurs in an estimated 40% of twin pairs during early language development. It arises primarily due to situational factors, specifically twins growing up in close proximity and using each other as linguistic models when adult model language exposure is limited or imperfectly acquired.
- supports: Autonomous languages of twins. (Acta geneticae medicae et gemellologiae 1987)
"Twins are regularly reported to invent languages of their own, unintelligible to others. These languages are known as autonomous languages, cryptophasia or idioglossia. Despite current belief, this is not a rare phenomenon. Autonomous languages exist in about 40% of all twins, but often disappear soon... The prototypical situation is one in which two or more close siblings (not necessarily twins) grow up closely together during the language acquisition period. If an adult model language is frequently absent, the children use each other as a model and acquire the language imperfectly." (abstract, passage verified)
pubmedfull study (doi) - supports: The early parenting of twins. (Military medicine 1996)
"The twin situation is triadic; the infants develop attachments to the primary care giver and their co-twin. Problems with this complex association include delayed onset of speech, decreased speech production, and cryptophasia (secret language) between twins." (abstract, passage verified)
pubmed