Often, when several norms are present and may be in conflict, individuals will display a self-serving bias, privileging the norm that best serves their interests. Xiao and Bicchieri (J Econ Psychol 31(3):456–470, 2010) tested the effects of inequality on reciprocating behavior in trust games and showed that—when inequality increases—reciprocity loses its appeal. They hypothesized that self-serving biases in choosing to privilege a particular social norm occur when the choice of that norm is publicly justifiable as reasonable, even if not optimal for one of the parties. In line with the literature on motivated reasoning, this justification should find some degree of support among third parties. The results of our experimental survey of third parties support the hypothesis that biases are not always unilateral selfish assessments. Instead, they occur when the choice to favor a particular norm is supported by a shared sense that it is a reasonable and justifiable choice.