AP,
Just to sign the potential bias for those who are following along at home, if 25% of dichorionic twins are really monozygotic, then studies that use chorionicity as a prozy for zygoticity would underestimate the correlation difference between MZ and DZ twins (by a factor of around 4/3, assuming sharing a placenta doesn't confound things).
And thus they would *underestimate* the role of heredity. Which is not usually the direction from which these studies are critiqued. Am I right?
Thanks for keeping it dispassionate.
UV
