According to Nash's theorem, your response in the Trust Game should be simply to keep whatever comes to you, even though you know some other person increased your wealth partly in the hope that you'd reciprocate. ... the unintended consequence of such "rational" behavior--that is, looking out for number one--is for both of you to miss the opportunity to gain by creating a larger pie, then sharing it. ... Yet with all deference to John Nash and his Nobel Prize, the Trust Game shows that rational self-interest is bupkis when it comes to real people. ... With large sums or small, in dollars or dinars, participants almost always behave with more trust and trustworthiness than the established theories predict they will.
Paul J. Zak
Book: The Moral Molecule -- The source of love and prosperity. 2012. Dutton; p 8