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Letters to the Editor
June 2002

Chorion Type and Twin Similarity for Child Psychiatric Symptoms

Arch Gen Psychiatry. 2002;59(6):562-564. doi:

Twin studies suggest significant genetic influence on all dimensions of childhood problem behavior measured with the Child Behavior Checklist (CBCL),1 which may increase with age.25 However, such twin studies have been criticized for assuming that monozygotic (MZ) and dizygotic (DZ) twins have similar prenatal environments.68 Monozygotic twins share the same chorion in most cases (monochorionic [MC]), whereas all DZ, and around a third of all MZ, twins are of the dichorionic (DC) type.6,9 The importance of these differences lies in the effects they have on placental development. Because the placenta is formed of chorionic tissue, DZ and MZ-DC twins will have separate placentas. Although these placentas may fuse macroscopically, fusion of 2 separate placentas only rarely results in direct vascular anastomoses in human twins,6,10,11 suggesting that their placentas remain functionally separate. However, MZ-MC twins share the same placenta. This results in competition between the twins for nutrition, which makes them substantially lighter at birth than other twins.6 In addition, sharing the same placenta may more often result in shared exposure to a prenatal environmental risk factor for psychiatric disorders, resulting in "environmental" concordance.1214 Earlier studies suggested higher concordance rates for schizophrenia15 and less intrapair differences for poor self-control, social incompetence, and internalizing symptoms16 in MC than in DC co-twins.

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