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Comparison of academic performance of twins and singletons in adolescence: follow-up study

BMJ 2006; 333 doi: (Published 23 November 2006) Cite this as: BMJ 2006;333:1095
  1. Kaare Christensen, professor1,
  2. Inge Petersen, mathematician1,
  3. Axel Skytthe, associate professor1,
  4. Anne Maria Herskind, paediatrician2,
  5. Matt McGue, professor3,
  6. Paul Bingley, associate professor4
  1. 1Danish Twin Registry, Institute of Public Health, University of Southern Denmark
  2. 2Paediatric Department, Odense University Hospital, Denmark
  3. 3Department of Psychology, University of Minnesota, USA
  4. 4Department of Economics, Aarhus School of Business, Denmark
  1. Correspondence to: K Christensen, Epidemiology, Institute of Health Sciences, University of Southern Denmark, DK-5000 Odense C, Denmark kchristensen{at}
  • Accepted 23 August 2006


Objectives To determine whether twins in recent cohorts show similar academic performance in adolescence to singletons and to test the effect of birth weight on academic performance in twins and singletons.

Design Follow-up study.

Setting Denmark.

Participants All twins (n=3411) and a 5% random sample of singletons (n=7796) born in Denmark during 1986-8.

Main outcome measures Test scores in ninth grade (age 15 or 16), birth weight, gestational age at birth, parents' age, and parents' education.

Results Ninth grade test scores were normally distributed, with almost identical mean and standard deviations for twins and singletons (8.02 v 8.02 and 1.05 v 1.06) despite the twins weighing on average 908 g (95% confidence interval 886 to 930 g) less than the singletons at birth. Controlling for birth weight, gestational age at birth, age at test, and parents' age and education confirmed the similarity of test scores for twins and singletons (difference 0.04, 95% confidence interval −0.03 to 0.10). A significant, positive association between test score and birth weight was observed in both twins and singletons, but the size of the effect was small: 0.06-0.12 standard deviations for every kilogram increase in birth weight.

Conclusions Although older cohorts of twins have been found to have lower mean IQ scores than singletons, twins in recent Danish cohorts show similar academic performance in adolescence to that of singletons. Birth weight has a minimal effect on academic performance in recent cohorts; for twins this effect is best judged relative to what is a normal birth weight for twins and not for singletons.


  • This is version 2 of the paper. In the first version the wording of the results section for participants with missing test scores was incorrect. It should have said that “The percentage with birth weights less than 1500 g was higher among twins without [not with] a test score than with [not without].”

  • Contributors: KC initiated the study, obtained funding, supervised the analyses, and was mainly responsible for writing the manuscript. He is guarantor. PB and IP helped develop the protocol, were responsible for creating and analysing the dataset, and helped write the manuscript. MMcG, AMH, and AS assisted with the protocol design, analysis, and writing of the manuscript.

  • Funding: University of Southern Denmark.

  • Competing interests: None declared.

  • Ethical approval: This study was approved by the Danish Data Protection Agency (case No 2000-54-0047).

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