A past focus of Kirschner's research has been the impact of the marital relationship on children's functioning and the common underlying characteristics of healthy marriages. Kirschner hypothesized the existence of a set of critical variables impacting children's well-being and social functioning, as well as the existence of a link between the quality of the marital relationship and everyday marital conflict and the way the spouses reared the child. Recent studies have validated this correlation. In a 2002 study of over 2,500 married parents with a child under 18 living at home, researchers found that marital conflict had a direct impact on children's problems, was related to more frequent use of harsh discipline, and increased the level of conflict between parents and adolescents. Kirschner's findings regarding the impact of marital conflict on children's adjustment are relevant to the "nature vs. nurture" controversy. Cummings and Davies reported similar results.