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

doi:10.1136/bmj.38633.594387.3A

BMJ 2005;331:1306 (3 December), doi:10.1136/bmj.38633.594387.3A (published 18 November 2005)

Paper

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

Georgina A Ronalds, research assistant¹, Bianca L De Stavola, reader in biostatistics¹, David A Leon, professor of epidemiology¹

¹ Department of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, London WC1E 7HT

Correspondence to: D A Leon david.leon{at}lshtm.ac.uk

Objectives To determine whether twins have lower IQ scores inchildhood than singletons in the same family and, if so, whetherdifferences in fetal growth explain this deficit.

Design Cohort study.

Setting Scotland.

Participants 9832 singletons and 236 twins born in Aberdeenbetween 1950 and 1956.

Results At age 7, the mean IQ score of twins was 5.3 pointslower (95% confidence interval 1.5 to 9.1) and at age 9, 6.0points lower (1.7 to 10.2) than that of singletons in the samefamily. Adjustment for sex, mother's age, and number of oldersiblings had little effect on these differences. Further adjustmentfor birth weight and gestational age attenuated the IQ differencebetween twins and singletons: the difference in mean IQ was2.6 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 thansingletons in the same family. This effect cannot be explainedby confounding due to socioeconomic, maternal, or other familycharacteristics, or by recruitment bias. The reduced prenatalgrowth and shorter gestations of twins may explain an importantpart of their lower IQ in childhood.

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