 There is some evidence that it affects the variance in IQ test findings among identical twins, that is, monochorionic identical twins display less IQ variance one from another than do dichorionic identical twins.There is weak evidence that monozygotic twins sharing a placenta have a higher concordance rate for schizophrenia than monozygotic twins with separate placentas.Sharing a placenta increases the risk for infection, and infection in pregnancy has been shown to be a risk factor for schizophrenia.