The cognitive cost of being a twin: evidence from comparisons within families in the Aberdeen children of the 1950s cohort study.

OBJECTIVES: To determine whether twins have lower IQ scores in childhood than singletons in the same family and, if so, whether differences in fetal growth explain this deficit. DESIGN: Cohort study. SETTING: Scotland. PARTICIPANTS: 9832 singletons and 236 twins born in Aberdeen between 1950 and 1956. RESULTS: At age 7, the mean IQ score of twins was 5.3 points lower (95% confidence interval 1.5 to 9.1) and at age 9, 6.0 points lower (1.7 to 10.2) than that of singletons in the same family. Adjustment for sex, mother's age, and number of older siblings had little effect on these differences. Further adjustment for birth weight and gestational age attenuated the IQ difference between twins and singletons: the difference in mean IQ was 2.6 points (-1.5 to 6.7) at age 7 and 4.1 points (-0.5 to 8.8) at age 9. CONCLUSIONS: Twins have substantially lower IQ in childhood than singletons in the same family. This effect cannot be explained by confounding due to socioeconomic, maternal, or other family characteristics, or by recruitment bias. The reduced prenatal growth and shorter gestations of twins may explain an important part of their lower IQ in childhood.