Child support used to be simpler. When current guidelines emerged in the 1980s they were envisioned for a couple divided by divorce. Most of the time the father paid the mother to support their children.

But times have changed. Although the typical family of three decades ago is still the most common, more than 40 percent of children today are born to unmarried parents, whose relationships are usually less stable than married couples and often sequential. A father paying child support today is more likely to have children in more than one household, and a mother is more likely to be owed support from more than one father.

A new study published in the December 2011 Social Service Review shows how this new family complexity poses unforeseen challenges for childsupport policies. In “Who Owes What to Whom? Child Support Policy Given Multiple-Partner Fertility,” Maria Cancian and Daniel Meyer of the University of Wisconsin describe these challenges, using child support data from Wisconsin. They also show how family complexity raises fundamental questions about child support guidelines.

For an unmarried father with three children, all with the same mother, child support payments are typically based on a percentage of a father’s income, with the amount per child declining for each additional child. In Wisconsin, a father supporting one child pays 17 percent of his income. If he has two children, he pays 25 percent. If three, 29 percent. The sliding scale takes into account economies of scale: a second child costs less to support than the first, and a third still less. It also makes the burden of child support manageable. Many fathers paying child support earn low incomes, and guidelines reflect an unwillingness to impose a greater burden than they can pay.

This approach works less well if a father has three children with different mothers. Then the father pays 17 percent for the first child, 17 percent of what’s left for the second, and 17 percent of what’s left after that for the third. The total in this case turns out to be 43 percent of his income. And the mothers receive differing amounts of support regardless of whether they have other children or are benefitting from any economies of scale.

Contradictions like these affect a growing number of families. The authors found that 31 percent of child support payments in Wisconsin involve complex families. The typical case is a mother with children from a single father who has had other children with two or more mothers.

And yet the authors find that any efforts to adjust child support guidelines to better suit complex families run up against practical limitations. The present system works for court officials because it doesn’t force them to alter previous child support orders when a parent has a new child; the courts simply calculate new obligations based on what’s already in place. Alternatives don’t do that.

The study also shows the difficulty of reconciling two important principles underlying child support policies. One is that all children in a family should be treated equally, wherever they reside and in whichever order they are born. The other, known as “hold-harmless,” is that a child should not suffer because his father has another child in a different household. In the case of complex families, the authors say, the two principles simply cannot be reconciled. “You can do one thing but not another,” Cancian says.

Indeed, the authors conclude that there is really no workable alternative to today’s child support policies. These policies still work for most families. But no one system can work well for every family.

Still, the authors say, it’s important to recognize the growing complexity of families. In part, they say, their study suggests the need to provide other kinds of support for poor families in addition to child support. “We have increasingly developed a system where we expect a private safety net, which includes child support, to be the primary source of support for economically vulnerable families,” Cancian says.

Maria Cancian and Daniel R. Meyer. 2011. “Who Owes What to Whom? Child Support Policy Given Multiple-Partner Fertility.” Social Service Review 85 (4): 587-617.