Final answer:
David Elkind based his main theory on the work of Jean Piaget who was influential in the field of child development, especially in the context of social interactions.
Step-by-step explanation:
The person who created the theories that David Elkind based his main theory off of is Jean Piaget. Jean Piaget was known for his pioneering work in child development, particularly regarding the role of social interactions in the development process. His theories highlighted how the self evolves through a balance between the mental representations of the world and the social experiences one encounters.
Elkind's work on adolescent development and related theories took Piaget's foundational ideas and expanded upon them to further explain how children and adolescents come to understand their own identities and the world around them.