In approval voting, voters indicate all alternatives acceptable to them, instead of indicating their most preferred alternative. This study sought to examine the hypothesis that, under certain conditions, approval voting systematically violates a rationality condition of social choice: the independence of irrelevant alternatives condition. Given a well-known psychological phenomenon, the contrast effect, it was hypothesized that sets of acceptable alternatives might be changed as a result of changes in unacceptable alternatives. This hypothesis was tested in a sample of undergraduates who were presented with fictional profiles of juvenile delinquents and then were asked whether they should be granted parole. The outcomes of approval voting changed merely as the result of a change in an alternative least preferred by all voters.