Article Figures & Data
Figures
Additional Files
Supplemental Material
Files in this Data Supplement:
- supplemental material - Supplemental Figures
Files in this Data Supplement:
The key assumption in this interesting paper by Fornito et al. is that a higher correlation among various neuroanatomical parameters in monozygotic twins compared to dizygotic twins can be simply interpreted as evidence for a genetic origin. This may, of course, be true, but it may not be. Monozygotic twins often have a very different intra-uterine experience than dizygotic twins, and this can, and does, affect their brain d...
The key assumption in this interesting paper by Fornito et al. is that a higher correlation among various neuroanatomical parameters in monozygotic twins compared to dizygotic twins can be simply interpreted as evidence for a genetic origin. This may, of course, be true, but it may not be. Monozygotic twins often have a very different intra-uterine experience than dizygotic twins, and this can, and does, affect their brain development, and many other features of their later lives.
To take a few of many substantive examples: Loos et al (2001) show that monochorionic monozygotic twins have markedly lower within-pair fibrinogen correlations than dichorionic monozygotic twins; McNeil et al. (2000) analyzed the impact of obstetric complications on the differences in brain anatomy between monozygotic twins; Poulsen and Vaag (2005) show significant differences in glucose metabolism between all twins and singletons, and between monozygotic twins and dizygotic twins.
There are also methodological difficulties in interpreting twin studies, which are not well considered by Fornito et al. An important paper by Turkheimer et al. (2005) describes the methodological issues in interpreting twin studies with great clarity. Their key point is that it is not possible to show that environmental and genetic influences partition as the SEM models assume, and in fact, that this is an untestable assumption of the models.
A recent NIH workshop (Bookman et al. 2011), published too late for this paper to make use of, outlines some routes towards a fuller understanding of how genes and environment interact to produce health outcomes.
Therefore, the conclusions of Fornito et al. about the genetic component in brain organization may or may not be correct. The authors assume that the traditional simplistic interpretation of complex twin study data is right, but it may not be, and they have not shown that it is.
References
Loos RJ, Beunen G, Fagard R, Derom C, Vlietinck R, Phillips DI. Twin studies and estimates of heritability. Lancet. 2001 May 5;357(9266):1445. PubMed PMID: 11360946.
Poulsen P, Vaag A. The intrauterine environment as reflected by birth size and twin and zygosity status influences insulin action and intracellular glucose metabolism in an age- or time-dependent manner. Diabetes. 2006 Jun;55(6):1819-25. PubMed PMID: 16731848.
McNeil TF, Cantor-Graae E, Weinberger DR. Relationship of obstetric complications and differences in size of brain structures in monozygotic twin pairs discordant for schizophrenia. Am J Psychiatry. 2000 Feb;157(2):203-12.
Turkheimer E, D'Onofrio BM, Maes HH, Eaves LJ. Analysis and interpretation of twin studies including measures of the shared environment. Child Dev. 2005 Nov-Dec;76(6):1217-33. PubMed PMID: 16274436.
Bookman EB, McAllister K, Gillanders E, Wanke K, Balshaw D, Rutter J, Reedy J, Shaughnessy D, Agurs-Collins T, Paltoo D, Atienza A, Bierut L, Kraft P, Fallin MD, Perera F, Turkheimer E, Boardman J, Marazita ML, Rappaport SM, Boerwinkle E, Suomi SJ, Caporaso NE, Hertz-Picciotto I, Jacobson KC, Lowe WL, Goldman LR, Duggal P, Gunnar MR, Manolio TA, Green ED, Olster DH, Birnbaum LS; for the NIH G X E Interplay Workshop participants. Gene-environment interplay in common complex diseases: forging an integrative model-recommendations from an NIH workshop. Genet Epidemiol. 2011 Feb 9. doi: 10.1002/gepi.20571. [Epub ahead of print] PubMed PMID: 21308768.
None declared