Research about the effect of cohabitation on child development should consider individual differences in child characteristics, variations in cohabitation arrangements, and the impact of family functioning. Studies should examine the impact of cohabitation on a parent's ability to care for the developmental needs of children, as well as on the perceptions of children about marriage and family relationships. In addition, child outcomes may be affected by the type of cohabiting union. For example, children living with their biological mother and her partner are more likely than children living with their biological father and his partner to have regular access to health care and better relationships with their nonresidential parent. Comparisons should be made between two biological parents who are married or cohabiting, as well as between step-married and step-cohabiting parents. Aspects of family functioning also can promote or inhibit healthy development. Although recent studies have found that harsh discipline decreases when a mother moves into a cohabiting or marital relationship, others have found that mothers in cohabiting relationships are more likely to be depressed. Children may experience difficulties as their parent's partner assumes a parental role. 15 references.