When I was 6, my 2-year-old brother started speaking, but in a language of his own devising. I was eager to enlist him in games (my favorite: “Slave”) and so quickly mastered his lexicon. My parents never managed to, despite their advanced degrees, and so our dinner table came to resemble a Camp David summit—two sides forced to use a translator to argue for their conflicting philosophies of life. “Eat peas!” “Throw peas!” “Feet on the floor!” “Feet in the tuna casserole!”

Siblings, and especially twins, have been inventing private languages since time immemorial, to little fanfare, but recently such ingenuity has captured the public’s imagination. This spring, a YouTube video of jabbering twins went viral, and even made it into the New York Times’ Well blog. The Washington Post recently celebrated a new play that revolves around a similar pair of girls and their “secret twin-speak.” Scientists, meanwhile, have spent the last few decades quietly building up a body of research into what they call “cryptophasia” or “twin language,” and they are of two minds about it. They find it fascinating, as a window onto the origins of human language, but they also worry that it hampers children’s development.

Twins are especially likely to maintain an invented language because they spend so much time together and are on the same developmental schedule. They imitate and reinforce each other’s early inventions, weakening each other’s incentive to learn the mother tongue. They spend less time communicating with parents and other adults, on average, than do nontwins, because they always have a ready playmate and because their parents are especially busy. Twin parents must change more diapers, sleep less, earn more, and parry the brilliant questions forever tripping off other parents’ tongues like, “Is it true that twins only have half a brain each?”

I called my mother recently, eager to learn more about my brother’s language. “Oh, he just had a speech impediment,” she said. That’s not unusual, I found out. When scientists first started researching cryptophasic kids, they too were disappointed to discover that kids mostly just mispronounced words or made inside references. For instance, their word for pasta might be “oliga” if they’d first had it at Olive Garden. As Queensland University of Technology psychologist Karen Thorpe explained to me by email, it’s like “the conversations between married couples where words are invented and abbreviated or restricted codes are used because full explanations are redundant.” In some cases, there’s no secret language at all, just the appearance of one. Those YouTube twins? Not cryptophasic, just really good at pretending to understand each other.

In rare cases, however, children do develop an entire language of their own, and amazingly, all full-blown twin languages spontaneously develop the same structure, regardless of the language spoken at home. Aarhus University linguist Peter Bakker told me that twin-language structure is unlike that of any established language, and its syntax doesn’t simply reflect the usual mistakes made by children. (Deaf children not taught sign language who invent their own also use this structure, by the way.) This “gives us a potential insight into the nature of language,” Bakker said, into mankind’s “first language,” now lost to history.

Twin languages are simple, just as simple as necessary, one might say. For one, they freely mix subjects, verbs, and objects, putting the most important item first in any context. In an Estonian study a child said, in his private language, “Again I foyer toward write come.” (Estonian grammar would have dictated, “I come again to the foyer to write.”) Negation appears at the sentence’s beginning or end, regardless of where it appears in the native language. Thus one Swiss child said, “Bobby, here drive no!” instead of, “Bobby, don’t drive here!” Verbs aren’t conjugated. There’s no way to locate things or events in time and space. And finally, twin languages almost never use pronouns, just proper names. Language can get simpler than this, but not much. (Bakker is a fan of the satirical 1982 film On Top of the Whale, in which scientists discover a primitive tribe whose language consists of only one phrase, which they apply to every object they come across. Rough translation: “It will be mine!”)

If language originated between just two people, it might well have looked like this: The seemingly universal twin-language structure is blissfully easy to use in one-on-one conversation. However, that first language would have had to evolve quickly to be useful to a larger community. Societies need “unambiguous ways to distinguish between subject and object,” Bakker says. “In the twin situation these can be dispensed with, but not in languages in which it is necessary to refer to events outside the direct situation.” So it’s hard to say for sure if twin language really does offer us that hoped-for picture of man’s first sentence. (“Voulez-vous coucher avec moi, ce soir?” is my guess.)

Linguist Bernard Comrie at UC-Santa Barbara cautions that research into the birth of language is still in its infancy. “First we were told that creole languages would provide us with insight into ‘first language,’ then when that didn’t pan out interest shifted to deaf sign language (also with mixed results)—I guess twin language will be the next thing,” he wrote me. Twin language is particularly difficult to test because children give it up quickly, except when they are very isolated. And you can’t just isolate kids on purpose—not anymore, anyway. Gone are the days when the pharaoh Psammetichus I could send two infants off to be raised by goats, or the Holy Roman Emperor Frederick II could forbid children’s caregivers from speaking to them.

We do know that kids generally stop using private languages (whether they’re “true” languages or based on mispronunciation) spontaneously or after a low-key intervention—a good thing psychologists say, because cryptophasia may harm children if it goes on for too long. My brother was placed in speech therapy when he started kindergarten, and made a full recovery, but other kids aren’t so lucky. Studies show that the longer kids practice cryptophasia the poorer they test later on. Is this cause or effect, you ask? It may be both. Cryptophasia could be the symptom of an underlying handicap, and a disincentive to getting with the program. In general, the more you wait to learn true language, the harder it gets. In the 1994 film Nell, a secluded adult woman (played by Jodie Foster) who speaks only a twin language tries to enter society. Peter Bakker says the twin language “is surprisingly realistic, given that very little was known at the time, but in the end of the film her English is too good.”

There’s another downside to cryptophasia: It hampers your socialization. Consider the British twins June and Jennifer Gibbons, born in 1963. They spoke English but also maintained a private lingo. They had few other friends growing up, partly because they moved often. In school, they were so relentlessly bullied by their peers that they eventually stopped speaking to anyone but each other and another sister.

The best-known case of children caught up in cryptophasia is that of Grace and Virginia Kennedy, who grew up in 1970s San Diego. Grace and Virginia, or Poto and Cabengo as they called each other, created a sensation when it was reported that at age 6 they still spoke no English, only a sophisticated private language of their own devising. “Dugon, haus you dinikin, du-ah,” said Grace in one typically opaque conversation. “Snup-aduh ah-wee diedipana, dihabana,” Virginia replied. They had suffered convulsions as infants, and their parents had concluded that the girls were permanently mentally handicapped. (In fact, both were later found to have relatively normal IQs.) As a result, they were kept indoors and away from other children, for their protection, and supervised mostly by their taciturn grandmother who left them to their own devices.

“Twin Girls Invent Own Language,” “Gibberish-Talking Twins,” “Like a Martian” crowed the media. However, scientists subsequently discovered that like most cryptophasics the girls had invented very little truly new vocabulary. They were just very badly pronouncing English and German, the two languages spoken at home. For example, instead of “Dear Cabengo, eat,” Grace once said, “Liba Cabingoat, it.” They were particularly hard to understand because they spoke rapidly and their pronunciations changed constantly. (The girls seem to have had at least 16 different ways to say “potato salad.”) Adding to their disappointment, the scientists couldn’t even use their hard-won knowledge of the girls’ babble to converse with them. When the girls heard it out of adults’ mouths, they couldn’t stop laughing.

Everyone involved projected onto the Kennedy girls his or her own prejudices, nightmares, and fantasies about childhood in America and the nature of language. This is well illustrated by the now-rare Jean-Pierre Gorin documentary, Poto and Cabengo, which the Criterion Collection may rerelease next year. In the film, their grandmother seems to consider their language a laudable effort to be seen, not heard. Their parents treat it as an opportunity for the girls to finally earn their keep, by attracting Hollywood dollars. The media’s wide-eyed fascination reflects the dawning of an age in which children are perceived to have secretly rich inner lives and adorable quirks. Gorin projects, too, as he admits. He can’t help but detect American philistinism, marveling at the girls’ under-stimulation—never once taken to the library, the bookstore, or the San Diego Zoo. He also indicts the American dream: The Kennedys moved constantly to make more money and upgrade their digs, to the detriment of their kids’ ability to form outside friendships.

The girls themselves, who eventually underwent speech therapy and learned English, haven’t commented publicly on their childhood. It was reported several years ago that both have found employment, but it is unskilled and their language skills still lag. It’s hard to know how much better the Kennedy twins would have fared if they had received therapy earlier. Some researchers advocate treating cryptophasia as early as possible, including Japanese psychologist Chisato Hayashi, who suggests placing cryptophasic children in intensive speech therapy and in separate classrooms at school. This is not yet universal practice. Oxford neuropsychologist Dorothy Bishop told me that twins generally get less intervention from speech therapists than nontwins. “People often assume that it’s normal for twins to have funny language, and so they don’t get a proper assessment and diagnosis. And then, when they are identified, they are often treated together as a unit, and so each gets half the attention of the professionals working with them.”

I suspect that it will be tricky to get more parents and professionals to treat twin language as a disease requiring a cure. It’s not just that people expect twins to talk funny and that twin parents are distracted. Cryptophasia makes twins seem like cute prodigies—by God, they’ve invented their own language! They’ve transcended the need for human society, at age 2! They’ve formed a bond closer than most anyone has with anyone! Their communication looks like an intensified form of a pleasure we’ve all experienced, that of secret communication, which is arguably an important part of every close relationship.

The French novelist Michel Tournier once wrote that “when two individuals laugh together—and only then—they come near to the mystery of cryptophasia. At such times they are using a pseudo-language, laughter, based on a common ground—stemming from a concatenation of shared experience—which, unintelligible in itself, has as its function to narrow the distance between their respective positions.” Who doesn’t like an inside joke? Who wouldn’t be tempted to live one’s whole life lost in one?