Letter: Twins take terror out of clone debate

Sir: Andrew Marr is right to highlight the breakthrough achieved by the Roslin Institute as a momentous change in the human condition ("Galileo, Copernicus - and now Dolly!", 26 February), and I share his belief that we will learn to "live with our own growing skill".

One reason for sharing that belief, without downplaying the magnitude of the change, is that we have experience of living with genetically identical human beings - at least since we stopped killing twins at birth.

Identical twins are just that - identical - at the genetic level, yet for all the extraordinary similarities of experience, attitudes and behaviour that are documented by my fellow psychologists, each twin develops into a unique individual. Conjoined twins, whether separated or not, further dramatically underline the simple point that emerges from "twin studies".

Our experience, attitudes and behaviour are strongly influenced by our genetic inheritance, but less and less is genetically determined, in the strict sense of "determined", as we move towards what distinguishes us as human beings.

I share a good deal of my own genetic make-up with my siblings, and were I an identical twin (or a clone), I would share the whole of it. But it seems to me that identical twins (and for that matter, in appropriate measure, siblings) give the lie to the "exposed film waiting to be developed" image of the sociobiologists in the sheer variety of choices and life- commitments displayed in their lives.

BRENDAN CALLAGHAN SJ

Principal, Heythrop College

University of London

London W8