Reciprocity

Reciprocity is a principle in which actions by one actor elicit a response from another that is roughly in kind. In everyday life, reciprocity underpins exchanges of favors, goods, or services, creating mutual obligation and social cohesion. In science and law, reciprocity describes symmetrical relationships in which processes or agreements can be reversed or honored in return. The term is used across disciplines to describe patterns of mutual exchange or mutual obligation.

In the social sciences, the norm of reciprocity is a foundational idea in social exchange and trust. It holds that people feel obliged to repay benefits received, which promotes cooperation. Cultures vary in how strictly they enforce it, and reciprocity can be voluntary or compelled by norms or sanctions. The concept appears in Marcel Mauss's The Gift and in subsequent theories of social exchange and reciprocity norms, which explain how gifting, favors, and retaliatory acts sustain social networks.

In biology, reciprocal altruism describes costly helping among unrelated individuals when the likelihood of future reciprocation makes the cost worthwhile. Proposed by Robert Trivers (1971), it requires stable relationships, recognition of individuals, memory, and an expectation of future returns. Vampire bats, cleaner fish, and certain primates are cited as examples.

In economics and game theory, reciprocity supports cooperative behavior and trust in repeated interactions. Strategies such as tit-for-tat reward cooperation and punish defection, enabling stable outcomes in repeated games. Reciprocity can also influence contractual and institutional design, encouraging reciprocity-based commitments and reputational mechanisms.

In physics and engineering, the reciprocity principle states that, under suitable conditions, exchanging the position of source and observer does not change the observed response. This symmetry underpins the behavior of antennas, acoustic devices, and optical systems, and is a key assumption in many theoretical models.