If 100 schizophrenics all have an identical twin, depending on which literature review you believe, between half and three-quarters of the twins will not also have the illness. Since they have the same genes, the only conceivable explanation for this discordance is the environment, probably nurture.
Differences start in the womb. In her remarkable book, Twins: From Fetus to Child, Alessandra Piontelli compared the intrauterine and subsequent lives of 15 identical with 15 non-identical twins. She shows that even prenatally, mothers are already dividing the twins into good and bad, intelligent and stupid.
Studies of discordant identical twins in which only one is schizophrenic reveal that sometimes one twin is born lighter and weaker as a result of differences during the pregnancy. The parents often use this as a coat hanger for their own preferences, showing a tendency to prefer the stronger one from birth.
As they develop, the future schizophrenic is more submissive, more shy, more neurotic and more obedient, tending to depend heavily on their stronger sibling, who is liable to be more outgoing, academically successful and lively.
The disturbed twin is more likely to identify with the parent with the most emotional pathology, and this parent also projects more negative feelings on to the future schizophrenic. The parents in these families tend to project their positive feelings on to one twin while the future schizophrenic becomes a dustbin for negativity and mystification.
In such cases, this parent is more liable to be the mother, who may be disturbed in a number of ways, such as being prone to depression or to incoherent thought patterns. Where the father is heavily involved with one of the twins, if he is sensitive and loving it can protect that twin from disturbance.
However, it was the father who was most negative in the cases of the identical Tim and George. At birth, Tim was felt to have his father's ears, to be more difficult and weaker. George was seen as more energetic and exploratory, more outgoing and sociable. Their mother, Jill, said: 'Each of your children are individuals even when they're identical. You can love them equally, but that doesn't mean your reactions to each of them is the same.' This was especially true of their father, Terry. A crucial issue for him had been a competitive struggle with his older brother Kevin, compared with whom he had always felt weaker, less successful and more plodding.
From birth onwards, Terry saw all the attributes that he felt his brother had and would have liked for himself in his son George, and the negatives that he felt about himself were all loaded on to Tim. Unfortunately for Tim, Jill also projected all her negativity on to him, and she was a very confusing mother. It was Tim who became schizophrenic.
If nurture really is as important as this, the implications are massive: we must create a society in which parents are given every conceivable help. As a goal, this should be streets ahead of individual and collective economic growth.