1. 1. Davis, J.M. & Svendsgaard, D.J. Nature 329,297 (1987). 2. Bellinger, D., Leviton, A., Waternaux, C., Needleman, H., & Rabinowitz, M. New Engl. J. Med. 316, 1037-1043 (1987). 3. Ward, N.I., Watson, R. & Bryce-Smith, D. Int. J. Biosoc. Res. 9(1), 63-81 (1987).