For the geneticist, or for anyone else, to make a speech in Jerusalem means a cultural effort that is extraordinary. Like every other researcher, the geneticist must locate his scientific investigations in the context of a truth which embraces nature and the history of man. Those of us who are concerned with twin studies, however, cannot simply leave our concrete and specific field to deal with theoretical themes; rather, we must extract from our research these concepts that bridge the gap between the particular truths we seek and that total truth, which man desires in order to resolve the mystery which surrounds his existence.

I would like to submit to the distinguished personalities who are honoring our Congress with their presence, as well as to my very dear colleagues, the idea that the best project for creating a link between our field of interest and the other interests of modern culture would be to survey the state of our knowledge about the personality of the human twin.

Therefore, I will not talk about the application of the method of twin studies, but about the identical twin, the monozygotic (MZ) twin individual. That is, I will not talk about those dizygotic (DZ) twins like Esau and Jacob, who were different. DZ twins are, of course, real twins, and about twice as frequent as the MZ ones. But I will refer only to the twin who has another twin identical to himself, and who represents the classical model of the phenomenon of twins.