But the expression of the brain as it grows is very dynamic, influenced by many factors outside the genetic, including itself, self-referentially. You could pr

But the expression of the brain as it grows is very dynamic, influenced by many factors outside the genetic, including itself, self-referentially. You could pretty easily raise one twin as an imbecile and the other as competent, purely by the means of changing their sensory inputs during the phase of brain development. If you had no scruples, of course.