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). doi:10.1136/bmj.38959.650903.7C 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
Official Date:	29 September 2006
Dates:	Date Event 29 September 2006 Published
Volume:	Vol.333
Number:	No.7578
DOI:	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]

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