Lev Vygotsky was a Russian psychologist born in 1886 who developed the Social Development Theory. His theory addressed cognitive development and the role of social interaction and culture in children's development. He believed that social interaction and relationships with supportive adults are essential for children to develop higher thinking skills. Vygotsky's zone of proximal development describes how adults should interact with children to extend their learning by providing scaffolding support that is gradually reduced as skills develop. His theory emphasizes that social interaction and culture play a central role in children's cognitive development.