If all the people in the world sat down to write letters, about 200 million of them would pick up their pens with their left hands, the rest with their right. What causes this strange asymmetry - asymmetry that extends even to the shape of the brain, and the position of some of the major organs? John Maddox discusses the biological basis of left- and right-handedness with Professor Michael Morgan of the University of Durham. and Professor Lewis Welpert of the Middlesex Hospital Medical School. Editor GEOFF deehan