FRIDAY, Nov. 25 (HealthDay News) -- Twins have lower IQ scores in childhood compared to their single-born brothers and sisters, according to a study published online Nov. 18 in the British Medical Journal.
David A. Leon, Ph.D., of the London School of Hygiene and Tropical Medicine in England, and colleagues studied the IQ scores of 9,832 singletons and 236 twins born between 1950 and 1956 in Aberdeen, Scotland. They also collected information on the children's birth weight, gestational age, the mother's age at delivery, the family's socioeconomic status and other family characteristics such as the number of older siblings.
At age 7 years, the researchers found that the twins had a mean IQ score that was 5.3 points lower than their singleton siblings, a difference that increased to 6.0 points at age 9.
The researchers concluded that reduced prenatal growth and shorter gestation may account for the IQ gap. They ruled out maternal, socioeconomic and family characteristics as possible causes.
"A cognitive deficit in twins of this magnitude is... clearly of long-term importance," the authors write. "However, whether the effects today are as large as we have reported requires study of a more contemporary cohort."
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