Kirschner also hypothesized the existence of common characteristics among relationally healthy or highly satisfied couples. She interpreted Tolstoy's opening line from Anna Karenina, "Happy families are all alike; every unhappy family is unhappy in its own way", as an observation that happy couples and families share characteristics. Kirschner asserted that relational success derived from a commitment by the spouses to help each other grow and develop. Experiments showed that high-satisfaction couples enjoyed spending time alone more than couples reporting medium or low relational satisfaction. Kirschner also found that successful couples and families learned to handle conflicts skillfully to prevent lingering resentment. Two decades of empirical research summarized by Gottman and Silver (1999), Olson and Flowers (over 6,000 couples studied, 1993) and a metanalysis by Bradbury, Fincham, and Beach (2000) have validated Kirschner's ideas and confirmed the relational importance of commitment, conflict resolution skills, desire to spend time with each other, sexual fidelity, and lack of violence.