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Unconditional in this sense means that no monetary or other compensation should be offered or demanded as a precondition of the act of sharing. It is thus to be seen in contrast to trade and exchange.

References:

Alvin Gouldner (1960). The Norm of Reciprocity: a Preliminary Statement. American Sociological Review, Vol. 25, No. 2, pp. 161-178.

Hornby, A. S. (2000). Share - Give some of yours. Oxford Advanced Learner’s Dictionary (6 Rev ed.). Oxford University Press, p. 1177.

Yamagishi, T., & Kiyonari, T. (2000). The Group as the Container of Generalized Reciprocity. Social Psychology Quarterly, 63(2), pp. 116–132.