Differences in fetal environment from one twin to another can explain the twin disparity. Corpus Callosum atresia is regulated by Testosterone (T). T receptors are also adrenaline receptors, so if one fetus is stressed at the time when the Corpus Callosum is breaking down, then the T doesn't get through because the receptors are occupied with adrenaline. Atresia is down regulated and now you have a male baby with a more female brain. Nucleus