The obvious advantage of joint physical custody is that living in both households allows a child to maintain a strong relationship with both parents. According to research, about half of all children in joint physical custody see both parents at least weekly, compared to one in 10 children in traditional custody arrangements (custodial mother, noncustodial father). "When both parents are available, children enjoy the unique guidance, discipline, and love of each parent," says Ms. Zinner. Moreover, "[c]hildren in joint custody may benefit materially, as child support is paid fully 75 percent of the time, compared to 46 percent in solo custody arrangements."

Children benefit when parental relations are cooperative and there is no extended legal wrangling. "When parents are reasonably satisfied with their custody plan, they are more likely to cooperate on a range of issues," Ms. Zinner says.

Children in shared custody have "normal time" with both parents. This avoids the situation where Mom, the custodial parent, competes with her former husband, a "Sunday Dad," who lavishes gifts and treats on his children in an effort to compensate for his absence.

Clinical social workers working with children with access to only one parent "found they expressed their anger in both subtle and direct ways. They were more depressed, withdrawn, and uncommunicative, and had more somatic symptoms."