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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 . . . [Full Text of this Article]

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