A neighbor helps you move a heavy table. You may later offer to collect a parcel for them, not because you signed a contract, but because their help changes what you feel willing or obliged to do. Reciprocity makes a relationship carry expectations across time.
Help can return directly from the person who received it, or indirectly because others learn that someone is a reliable contributor. A reputation for helping can make people beyond the original recipient more willing to cooperate.
Reciprocity does not require an immediate or identical return. A community can exchange different forms of help according to people’s abilities and circumstances. Insisting on exact equivalence can make ordinary generosity feel like accounting.
But vague obligations can also become coercive. A powerful person may offer assistance and later treat it as entitlement to loyalty. Someone with fewer resources may find that they can never repay what others count as a debt. Reputation can amplify unfair judgments if people see the refusal but not the circumstances behind it.
The mechanism works through expectations, so its character depends on how those expectations are understood. Help that leaves people able to decline a later request creates a different relationship from help that quietly purchases control over them.