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...

We examine the effect of single-sex classes on the pass rates, grades, and continued enrollment of students in a coeducational university. We randomly assign students to all-female, all-male, and coed classes and, therefore, get around the selection issues present in studies on single-sex...