Is there still a cognitive cost of being a twin in the UK?

Objective: To investigate whether twins still perform below singletons in cognitive ability tests and in national educational attainment examinations in a recent UK cohort of 11-year-olds. Design: Population-based cohort Study, using a validated method of twin extraction. Setting: English state schools. Participants: 178,599 pupils (mean age 11 years) attending English state schools in 2004. representing 93% of local educational authorities. Main outcome measures: Verbal, quantitative and nonverbal reasoning measured by the Cognitive Abilities Test - Third Edition (CAT3), and educational attainment in English. maths and science from Key Stage 2 national Curriculum tests. Results: Twin-singleton differences in cognitive ability were of negligible effect size. Mean performance of twins was 1% of a SD below singletons on general cognitive ability, 3% lower on verbal reasoning, and less than 1% of a SD lower on quantitative and nonverbal reasoning. with and without adjustment for sex. Almost identical proportions of twins and singletons attained the expected standards of educational performance in English (80.4% versus 80.5%, respectively), maths (77.4% versus 77.3%), and science (90.1% versus 88.8%). There were no differences in rates of twins and singletons achieving higher or lower performance levels. Conclusion: The cognitive cost of being a twin reported in previous cohorts of UK children no longer exists in a large. contemporary and population-representative sample. (C) 2009 Elsevier Inc. All rights reserved.