Social Penetration Theory proposes that relationships develop through reciprocal self-disclosure, from nonintimate to intimate topics over time. The theory uses an onion analogy - relationships start broadly and become deeper. There are costs and rewards to relationships, with rewards outweighing costs leading to stronger bonds. Relationships are more impacted by costs and rewards early on, and past positive experiences can help relationships withstand future conflicts. The theory is relatively testable but definitions of intimacy may vary, and it has produced further studies examining culture and relationship types.