Final answer:
Erik Erikson is the theorist who believed each stage of the lifespan had its own developmental theme and psychosocial crisis, focusing on the social aspects of personality development throughout the entire lifespan.
Step-by-step explanation:
The theorist who believed each stage of the lifespan had its own developmental theme and psychosocial crisis was Erik Erikson. Erikson's theory, influenced by but distinctly different from Freud's psychosexual stages, proposed that personality development occurs throughout one's entire lifespan, consisting of eight stages starting from birth.
Contrary to Freud, who emphasized sexual nature in development, Erikson focused on the social nature of development and believed that interpersonal relationships are key at each stage. Each of Erikson's stages is characterized by a specific psychosocial crisis that must be resolved for healthy personality development to take place.
These stages highlight the importance of achieving competence in various areas of our lives, where the resolution of each crisis leads to the development of a sense of competence and contributes to a healthy personality. Failure to successfully negotiate these crises can result in a sense of inadequacy.
Erikson emphasized that this development process continues over the entire lifespan, marking his theory as a vital contribution to the domain of developmental psychology, particularly in understanding how our interactions with others shape our sense of self, or ego identity.