Objectives To determine whether twins in recent\ud cohorts show similar academic performance in\ud adolescence to singletons and to test the effect of\ud birth weight on academic performance in twins and\ud singletons.\ud Design Follow-up study.\ud Setting Denmark.\ud Participants All twins (n = 3411) and a 5% random\ud sample of singletons (n = 7796) born in Denmark\ud during 1986-8.\ud Main outcome measures Test scores in ninth grade\ud (age 15 or 16), birth weight, gestational age at birth,\ud parents’ age, and parents’ education.\ud Results Ninth grade test scores were normally\ud distributed, with almost identical mean and standard\ud deviations for twins and singletons (8.02 v 8.02 and\ud 1.05 v 1.06) despite the twins weighing on average\ud 908 g (95% confidence interval 886 to 930 g) less\ud than the singletons at birth. Controlling for birth\ud weight, gestational age at birth, age at test, and\ud parents’ age and education confirmed the similarity of\ud test scores for twins and singletons (difference 0.04,\ud 95% confidence interval − 0.03 to 0.10). A significant,\ud positive association between test score and birth\ud weight was observed in both twins and singletons, but\ud the size of the effect was small: 0.06-0.12 standard\ud deviations for every kilogram increase in birth weight.\ud Conclusions Although older cohorts of twins have\ud been found to have lower mean IQ scores than\ud singletons, twins in recent Danish cohorts show\ud similar academic performance in adolescence to that\ud of singletons. Birth weight has a minimal effect on\ud academic performance in recent cohorts; for twins\ud this effect is best judged relative to what is a normal\ud birth weight for twins and not for singletons
To submit an update or takedown request for this paper, please submit an Update/Correction/Removal Request.