Intended for healthcare professionals


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

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

Competing interests:
Taiwo Ajayi is himself one of a pair of Mono-zygotic Twins.

Competing interests: No competing interests

09 October 2006
Taiwo Ayodele Ajayi
Staff Grade Psychiatrist
Kent & Medway Substance Misuse Service, 4 Manor Road Chatham Kent ME4 6AG