Criticism #2: MZ twins experience life more similarly starting as fetuses. DZ twins are “dichorionic,” meaning that they have separate placentas. In contrast, 75 percent of MZ twins share one placenta (i.e., are “monochorionic”).* Thus most MZ twin fetuses share maternal blood flow more than do DZ twins, and thus are exposed to more similar levels of maternal hormones and nutrients. If that isn’t recognized, greater similarity in MZs will be misattributed to genes.
