The growth and development of twins compared with singletons at ages 9 and 11.

The growth and development of 9 and 11-year old twins who have been studied at birth, and at ages 3, 5, and 7 years was compared with that of a large group of singletons. The twins were not significantly different to the singletons in language development, reading, spelling or Performance IQ. They remained significantly shorter, lighter and had smaller head circumferences. This difference was shown to have been fairly consistent from age 3. The twins had significantly lower Verbal and Full Scale IQ, but in view of a lack of disadvantage in educational progress this was not considered to be of any practical importance.