Centorrino, Samuele, Djemaï, Elodie, Hopfensitz, Astrid, Milinski, Manfred and Seabright, Paul (2011) Smiling is a Costly Signal of Cooperation Opportunities: Experimental Evidence from a Trust Game. TSE Working Paper, n. 11-231, Toulouse

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Abstract

We test the hypothesis that "genuine" or "convincing" smiling is a costly signal that
has evolved to induce cooperation in situations requiring mutual trust. Potential
trustees in a trust game made video clips for viewing by potential trusters before the
latter decided whether to send them money. Ratings of the genuineness of smiles vary
across clips; it is difficult to make convincing smiles to order. We argue that smiling
convincingly is costly, because smiles from trustees playing for higher stakes are
rated as significantly more convincing, so that rewards appear to induce effort. We
show that it induces cooperation: smiles rated as more convincing strongly predict
judgments about the trustworthiness of trustees, and willingness to send them money.
Finally, we show that it is a honest signal: those smiling convincingly return more
money on average to senders. Convincing smiles are to some extent a signal of the
intrinsic character of trustees: less honest individuals find smiling convincingly more
difficult. They are also informative about the greater amounts that trustees playing for
higher stakes have available to share: it is harder to smile convincingly if you have
less to offer.