From Herbert Gintis:

Re: Tit-for-tat: birds found to repay wartime help (July 6): you say that reciprocal altruism had previously been found in nonhuman species. This is incorrect, I think. It probably has not been seen prior to these studies.

See Peter Hammerstein, “Why Is Reciprocity So Rare in Social Animals?”, in Peter Hammerstein (eds.) Genetic and Cultural Evolution of Cooperation (Cambridge, MA: The MIT Press, 2003):83-93; and Kevin C. Clements and David W. Stephens, “Testing Models of Non-kin Cooperation: Mutualism and the Prisoner’s Dilemma”, Animal Behaviour 50 (1995):527-535; and W. Stephens, C. M. McLinn and J. R. Stevens, “Discounting and Reciprocity in an Iterated Prisoner’s Dilemma”, Science 298 [13 December] (2002):2216 2218.

Herbert Gintis
Santa Fe Institute and
Central European University
15 Forbes Avenue, Northampton, MA