"Cryptophasia" is the term for the secret languages that apparently 40 percent of identical twins develop. Nautilus investigates:

The most famous example of cryptophasia is identical twins Ginny and Grace Kennedy from California.

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http://feeds.boingboing.net/~r/boingboing/iBag/~3/hKPSR-aodp0/secret-languages-of-twins.html