"If you're going to separate a child from one of his or her parents for most of their childhood, it's only fair to say why," said Holstein of the National Parents Organization, who lives in the state.
Proponents of shared parenting argue that current laws treat fathers unfairly. But gender bias in custody cases may be more perception than reality.
While mothers are custodial parents 82.5 percent of the time, it may just be because fathers aren't asking for that job. A study in Massachusetts found that fathers who actively sought primary or joint custody obtained it more than 70 percent of the time.
In at least one state, specialists say recent changes to custody laws have made a difference. The Arizona Legislature passed a policy statement in 2011 in support of shared parenting and, in 2012, changed custody law to mirror the statement. This changed the culture of the court system, at least in Maricopa County, according to both Fabricius of the state university and Annette Burns, a lawyer there who specializes in family law.
The perceptions of parents have changed, too, Burns said. Now, both parents know they are likely to get a significant amount of time with their child.