This book presents the most comprehensive review of research regarding personality and motivational differences in persons with mental retardation. From the personal commentary of Edward Zigler, H. Carl Haywood, and Harvey N. Switzky, the book summarizes the classical work of the Yale and Peabody-Vanderbilt School over the last 40 years. A sampling of new directions in research is provided, including work on self-determination theory and practice; decision making; direct and indirect effects of genetic mental retardation syndromes on personality; personality and psychopathology in genetic mental retardation syndromes; a new theory of information processing linking cognition, motivation, and performance; and a sensitivity theory of motivation. This definitive work presents older and evolving newer models and applications to the field in order to demonstrate the power of motivational variables in understanding the behavior of persons with mental retardation. The purpose is to enhance the quality of life in persons with mental retardation and other developmental disabilities.