Copyright 2002 American Medical Association. All Rights Reserved.
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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.2- 5
However, such twin studies have been criticized for assuming that monozygotic
(MZ) and dizygotic (DZ) twins have similar prenatal environments.6- 8 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.12- 14
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.
Wichers MC, Danckaerts M, Van Gestel S, Derom C, Vlietink R, van Os J. Chorion Type and Twin Similarity for Child Psychiatric Symptoms. Arch Gen Psychiatry. 2002;59(6):562-564. doi: