We establish and maintain ongoing relationships to have more reliable and more predictable ways of meeting needs. We also maintain friendly relationships with people, who differ from friends by friends having voluntary interaction outside of formal roles. We have reference groups to whom we look for norms of appropriate behavior and comparisons to evaluate ourselves. Relationships vary in intimacy and in status, which often involves differences in power. Identities are defined in terms of relationships to specific others, such as family members, and in regard to groups of people, such as ethnicity. Relationships develop through a process of incremental exchange, in which rewards accepted and reciprocated lead to increased rewards through the development of trust.