What is not so widely known is that this special relationship often acts as brake on twins' intellectual development. As they are partly isolated in their own private world, twins communicate less with adults than do other children. The verbal ability of a four-year-old twin is typically six months behind that of a non-twin. The problem can be particularly severe in a deprived home, a one-parent family for example, where there is little stimulation for children anyway.
Such children, while capable of mutual comprehension in a private language, often remain in comprehensible to outsiders and thus at a severe educational disadvantage. The only solution to the problem, cruel though it may seem, is to separate the twins thus forcing them to acquire ordinary speech helped and guided by sympathetic parents and teachers.
Many people don' t know that ______.

A. twins understand each other very well
B. twins are slow to learn to talk
C. twins are unlikely to do less well at school than other children
D. there exists more communication between twins