1. G. E. McClearn and F. Ahern, Center for Developmental and Health Genetics, The Pennsylvania State University, University Park, PA 16802, USA.
2. B. Johansson and S. Berg, Institute for Gerontology, University College of Health Sciences, Jönköping, S-551 11, Sweden.
3. N. L. Pedersen, Institute for Environmental Medicine, Karolinska Institute, Stockholm, S-17177, Sweden.
4. S. A. Petrill and R. Plomin, Social, Genetic and Developmental Psychiatry Research Centre, Institute of Psychiatry, De Crespigny Park, Denmark Hill, London, SE5 8AF, UK.