M. Weller, T. Fundudis and I. KolvinIn: The Scientific Basis of Psychiatry 1992With thanks to Elsevier for their support for this project and for giving permission to reproduce this chapter. However we have been unable to locate a copy of this chapter. Please contact us if you have a copy or know where to find one.Cite as: Reprinted from The Scientific Basis of Psychiatry, M. Weller and M. Eysenck (eds.), "Human Development", M. Weller, T. Fundudis, I. Kolvin. (©W.B Saunders, 1992).