Researchers studied 9,832 single-born children and 236 twins born in Aberdeen, Scotland between 1950 and 1956, using a previous child development survey as a base. They also gathered further information on mother's age at delivery, birth weight, at what stage of the child's gestation they were born, their father's occupational social class, and information on other siblings.
They found that at age seven, the average IQ score for twins was 5.3 points lower than that for single-born children of the same family, and 6.0 points lower at age nine.
The study also showed that taking into account factors such as the child's sex, mother's age, and number of older siblings made little difference to the IQ gap.
Despite advances in recent years in obstetric practice and neonatal care, the authors argue that the likely explanation is because some twins have a shorter length of time in the womb than other children and are prone to impaired fetal growth.