In the past 30 years Americans have become more likely to live alone and less likely to join groups—ranging from PTAs to political parties to casual sports teams. Several studies hint that this type of isolation can take a toll on people’s attitudes toward others. Steve Duck of the University of Iowa has found that socially isolated, as compared with integrated, individuals evaluate others less generously after interacting with them, and Kenneth J. Rotenberg of Keele University in England has shown that lonely people are more likely to take advantage of others’ trust to cheat them in laboratory games.