There has been a vigorous debate in recent time, about heredity versus environment and how they coalesce to create personalities, behaviors, and psychopathology that appeared unique to each person and influenced their developmental process. This book therefore explores and used different methodological approach to lifespan development to measure and analyze the interplay of genes and environments on human behavior and underline how the interaction contributes to behavioral changes and continuity. The book planned to increase our knowledge and understanding of the interaction between heritable and ecological factors in the path of behavioral growth and patently recognize etiological mechanism that support the assumption that a particular hereditary or environmental condition ultimately lead to behavioral feature and changes during development. The book should help shed some light on the continuous source of person uniqueness in personality development,and should be especially useful to professionals in human development or any one else whose interest is in human behaviour.