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RESEARCH: Kaare Christensen, Inge Petersen, Axel Skytthe, Anne Maria Herskind, Matt McGue, and Paul Bingley Comparison of academic performance of twins and singletons in adolescence: follow-up study BMJ 2006; 333: 1095 [Abstract] [Full text]

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Twin Cohort undifferentiated

Taiwo Ayodele Ajayi   (9 October 2006)

Genetic contributions to intelligence differ between monozygotic and dizygotic twin pairs

Kenneth A Hoekstra, PhD   (30 November 2006)

Twin Cohort undifferentiated 9 October 2006
Taiwo Ayodele Ajayi, Staff Grade Psychiatrist Kent & Medway Substance Misuse Service, 4 Manor Road Chatham Kent ME4 6AG Send response to journal: Re: Twin Cohort undifferentiated	I note that though this study appears to have a fairly large cohort of twins it fails to differentiate between mono-zygotic ( identical) and di-zygotic (unidentical)twins in the cohort. Given that IQ has been highly associated with genes I deem this lack of differentiation a flaw of the study.The fact that there was no mention of controlling for this important confounder is in my opinion another weakness. It would have been interesting to see the results of the IQs of singletons compared with an identical twin cohort and an unidentical twin cohort independently rather than comparism with a mixed bag cohort of twins. Competing interests: Taiwo Ajayi is himself one of a pair of Mono-zygotic Twins.
Genetic contributions to intelligence differ between monozygotic and dizygotic twin pairs 30 November 2006
Kenneth A Hoekstra, PhD, Assistant Professor of Pathology Portland, Oregon 97230 Send response to journal: Re: Genetic contributions to intelligence differ between monozygotic and dizygotic twin pairs	Dear Editor, In the article by Christensen et al. titled 'Comparison of academic performance of twins and singletons in adolescence: follow-up study' the authors report that twins show similar academic performance in adolescence to singletons, and birth weight showed little to no affect on academic performance (1). In support of previous comments, this study however has failed to identify (include) the more important intelligence differences which have recently been identified between monozygotic and dizygotic twin pairs (2,3). 1. Christensen K, Petersen I, Skytthe A, Herskind AM, McGue M, Bingley P. Comparison of academic performance of twins and singletons in adolescence: follow-up study. BMJ. 2006: 333(7578):1095. 2. Hulshoff Pol HE, Schnack HG, Posthuma D, Mandl RC, Baare WF, van Oel C, van Haren NE, Collins DL, Evans AC, Amunts K, Burgel U, Zilles K, de Geus E, Boomsma DI, Kahn RS. Genetic contributions to human brain morphology and intelligence. J Neurosci. 2006: 26(40):10235-42. 3. Potter GG, Plassman BL, Helms MJ, Foster SM, Edwards NW. Occupational characteristics and cognitive performance among elderly male twins. Neurology. 2006: 67(8):1377-82. Competing interests: None declared