Publication Date



Open access

Embargo Period


Degree Type


Degree Name

Master of Science (MS)


Psychology (Arts and Sciences)

Date of Defense


First Committee Member

Michael E. McCullough

Second Committee Member

Debra Lieberman

Third Committee Member

William A. Searcy


Evidence suggests that smiles can function as signals of cooperative intent—by producing a smile, the smiler can expect to receive benefits from perceivers and perceivers can expect a return from smilers. However, this type of signal is seemingly susceptible to the evolution of cheats who smile in the absence of cooperative intent, thereby receiving the benefits of smiling without paying the costs of cooperation. If smiles were to maintain reliability over evolutionary time, some mechanism(s) must have prevented the evolution of cheats. Researchers have not yet developed a paradigm to directly test why smiles might have maintained reliability over evolutionary time. This experiment was the first to assess whether smiles might have maintained reliability due to receiver-dependent costs associated with smiling without cooperative intent. 385 participants played a Trust Game with a confederate who was either smiling or not smiling, and who then behaved either fairly or unfairly. Subsequently, participants provided self-report levels of anger and happiness. Finally, participants had an opportunity to punish, reward, or do nothing to the confederate. Results showed that those treated unfairly by a smiler did not become angrier toward, nor did they inflict more costs on, the confederate than did those treated unfairly by a non-smiler. There are some methodological concerns worth addressing before dismissing the hypothesis; most notably, it needs to be addressed why participants in this study were not more likely to trust smilers than non-smilers, as has been shown in prior research.


Smiles; Cooperation; Signal; Signaling Theory; Facial Expression; Evolution