A method for comparing the development of monozygotic twins in which one is given regular practice or training on a specific ability or a range of them, while such experiences are withheld from the other twin. If practice does not make a difference, then it taken as evidence that the environment plays little role in development and thus it is genetically determined. In its original form, as employed by Arnold Gesell and Myrtle B. McGraw before WWII, this method would not now meet with ethical approval, but it finds an echo in behavior genetics when attempting to dissect hereditary and environmental contributions to the development of a range of traits by means of comparing monozygotic and dizygotic twins using a range of sophisticated statistical techniques.
See Behavior genetics, Dizygotic twins, Hereditary, Monozygotic twins