Another important focus in Damon’s work has been his study of moral exemplars. His first book on this subject, "Some Do Care: Contemporary Lives of Moral Commitment" (1992), written with developmental psychologist Anne Colby, opened up a new perspective on moral development and has been widely cited and built upon in the field. Damon and Colby studied individuals who had shown “a sustained commitment to moral ideals or principles” over many decades of their lives. The authors found a number of qualities that were consistent across the entire group of exemplars, including a sense of certainty about their core moral beliefs, a positive attitude toward hardship and challenge, receptivity toward new ideas and goals, a lifelong capacity for moral growth, and a strong integration of their moral values into their sense of self. This book promoted the "moral identity" theory, which maintains that moral commitment and moral action are a function of how important morality is to a person's sense of self. This theory is distinct from theories that emphasize the primary importance of moral reasoning skills (Lawrence Kohlberg), emotions (Jonathan Haidt), or cultural background (Richard Shweder). This work also is known for its case study and interview methodology, which some believe to be more evocative than experimental studies. Damon and Colby followed up their "Some Do Care" study of living moral exemplars with some historical case studies of 20th century moral exemplars, described in their 2015 book "The Power of Ideals".