For a long time now, parents have claimed their twins have their own special language, understood only by the two of them. Scientists and laymen alike have long been fascinated by the idea of a secret and exclusive language developed by and used only by twins. The phenomenon is known as twin language, cryptophasia, or idioglossia.
Most of the time, the twins are not speaking a new or separate language. Rather, one twin is compensating for the speech deficits of the second twin. It is known that premature babies and low birth weight babies are subject to a delay in developing speech. Twins are often premature and low birth weight. In general, one twin seems to have developed speech at a faster rate than the other twin, but in order to help his sibling, he adapts his speech to that of his twin, mimicking his delayed and flawed speech. He wants to make sure his twin understands him, so he exaggerates these speech errors and incorporates them into his dialogue with his sibling. Sometimes both twins have delayed speech and their shared language will incorporate the flaws of both twins. While the twins make perfect sense to each other, it sounds like gibberish or some kind of foreign language to you.
This happens a lot, since about 60% of twins and over 90% of higher multiple births are born premature. Each additional baby decreases the length of gestation. A single birth pregnancy lasts in general around 39 weeks. In twin births, that number drops to around 36 weeks. Triplets, quadruplets, and quintuplets can range from a respective 32 to 29 weeks total gestation.
There are other factors that may lead to twin language, such as less one on one communication with parents plus the more developed non-verbal communication skills so often shared by twins. Their relationship is often closer than that between other siblings or between child and parent. Perhaps the continued modified language that remains incoherent to others is just a manifestation of the special nature of the twin relationship.
Research seems to indicate that twin language isn't what it seems, but that doesn't rule out the possibility that some twins make up their own words and codes. In other cases, one twin takes over for the other twin and acts as spokesman as in the case of Jane and Leslie Abrams, adult twins in their 30's. Jane is never at a loss for words unless Leslie is in town. When Leslie's around, Jane clams up. When asked why, Jane demurs, "Leslie says it so much better than I could ever do. It just makes sense to let her talk for both of us."