Christensen, Kaare, Petersen, Inge, Skytthe, Axel, Herskind, Anne Maria, McGue, Matthew and Bingley, Paul. (2006) Comparison of academic performance of twins and singletons in adolescence : follow-up study. BMJ, Vol.333 (No.7578). ISSN 0959-535X

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Abstract

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.

Item Type:	Journal Article
Subjects:	L Education > LB Theory and practice of education
Divisions:	Faculty of Social Sciences > Economics
Library of Congress Subject Headings (LCSH):	Birth weight, Academic achievement -- Testing, Twins -- Ability testing, Teenagers -- Ability testing
Journal or Publication Title:	BMJ
Publisher:	BMJ Group
ISSN:	0959-535X
Date:	29 September 2006
Volume:	Vol.333
Number:	No.7578
Identification Number:	10.1136/bmj.38959.650903.7C
Status:	Peer Reviewed
Access rights to Published version:	Restricted or Subscription Access
Funder:	Syddansk universitet [University of Southern Denmark]
References:	1 Hack M, Taylor HG, Drotar D, Schluchter M, Cartar L, Andreias L, et al. Chronic conditions, functional limitations, and special health care needs of school-aged children born with extremely low-birth-weight in the 1990s. JAMA 2005;294:318-25. 2 Shenkin SD, Starr JM, Deary IJ. Birth weight and cognitive ability in childhood: a systematic review. Psychol Bull 2004;130:989-1013. 3 Record RG, McKeown T, Edwards JH. An investigation of the difference in measured intelligence between twins and single births. Ann Hum Genet 1970;34:11-20. 4 Deary I, Pattie A, Wilson V, Whalley L. The cognitive cost of being a twin: two whole-population surveys. Twin Res Hum Genet 2005;8:376-83. 5 Ronalds GA, De Stavola BL, Leon DA. The cognitive cost of being a twin: evidence from comparisons within families in the Aberdeen children of the 1950s cohort study. BMJ 2005;doi:10.1136/bmj.38633.594387.3A (published 18 Nov 2005). 6 Petersen JK. The Danish demographic database: longitudinal data for advanced demographic methods. Research Report 15. Odense: Danish Center for Demographic Research, University of Southern Denmark; 2000. 7 Andersen TF, Madsen M, Joergensen J, Mellemkjaer L, Olsen JH. The Danish national hospital register: a valuable source for modern health sciences. Dan Med Bull 1999;(46):263-8. 8 Danish Ministry of Education. Tests, assessment and teaching: an overall evaluation of compulsory school completion assessment and test scores May-June 2004 [in Danish]. Copenhagen: Danish Ministry of Education, 2000. 9 Skytthe A,Kyvik K, Holm NV,Vaupel JW, Christensen K. The Danish twin registry: 127 birth cohorts of twins. Twin Res 2002;5:352-7. 10 Bingley P, Jensen VM, Walker I. School resources, curriculum design and test scores at the end of compulsory schooling in Denmark.Working paper. Aarhus: Aarhus School of Business, Economics Department, 2005. 11 Naglieri JA, Bornstein BT. Intelligence and achievement: just how correlated are they? J Psychoeduc Assess 2003;21:244-60. 12 Bartels M, Rietveld MJH, van Baal GCM, Boomsma DI. Heritability of educational achievement in 12-year-olds and the overlapwith cognitive ability. Twin Res 2002;5:544-53. 13 Wilcox AJ. Invited commentary: the perils of birth weight—a lesson from directed acyclic graphs. Am J Epidemiol 2006 Aug 24; [Epub ahead of print]. (Accepted 23 August 2006)
URI:	http://wrap.warwick.ac.uk/id/eprint/4301

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