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(June 2011) The most disadvantaged U.S. parents are also most likely to have children with more than one partner, creating complex family relationships and potentially exacerbating poverty, according to Marcia Carlson, a sociology professor at the University of Wisconsin, Madison.

As part of PRB’s 2010-2011 Policy Seminar series, Carlson examined the magnitude and implications of adults having children with more than one partner, a trend called “multi-partner fertility” by demographers and family researchers. She reported findings from the Fragile Families and Child Wellbeing Study, which is following a nationally representative group of nearly 5,000 urban children born in the late 1990s and their parents.

Among the married parents surveyed, 21 percent had a child by another partner. That is, the mother, father, or both already had a child with someone else. Among the unmarried parents, 63 percent already had a child with another partner. This means that more than half of children with unmarried parents are “born into families with at least one half-sibling,” said Carlson. By the time those children celebrated their fifth birthdays, 71 percent had a half-sibling, reflecting the instability of unwed relationships. (The Fragile Families study found that two out of three couples had split up within five years after an unmarried birth.)

In 2010, 41 percent of U.S. births were to unmarried parents. As a result, the share of U.S. children whose family lives are shaped by multi-partner fertility is substantial. Carlson also reported the findings of a variety of other studies indicating that as many as one in five children have a half-sibling, and as many as one in three mothers on welfare have a child with more than one partner.

Overall, her findings suggest that having children with more than one partner is most common among parents who are young, African American, lived with one parent at age 15, and have lower education levels. The fathers were more likely to have spent time in jail and the birth was more likely to be the result of an unintended pregnancy. “The least advantaged couples are most likely to have children with multiple partners,” she said. “We are just beginning to get a handle on what this means for kids and families.”

Complex Family Dynamics, Diminished Investment in Children

Multi-partner fertility creates households that may include children who are “hers, his, and ours.” Children relate to their own parents, their parents’ new partners, and the parents and new partners of their half-siblings. Obligations are blurred when parents do not live with their biological children and adults share households with children they are not tied to by blood or through marriage, she pointed out.

One concern is the impact on parents’ investment in children. Carlson reported that fathers who go on to have a child with a new partner tend to reduce the time and money they spend on previous biological children. “Some people have called this ‘swapping families,’ the fathers have moved on in a certain way,” Carlson said. “There’s less incentive to invest in your kids when you don’t live with them; you don’t know how the money you give is spent; you are not involved with your children daily.”

Carlson’s study found that parents are less able to cooperate in child rearing after the father has another child with another woman, that “the level of supportive co-parenting goes down,” and the father is less involved.

Other researchers analyzing the data find that conflict and distrust surround men trying to be involved with children by different partners. Current partners are often suspicious that he is “not just seeing the child and has a relationship with the other women,” Carlson explained.

And there is early evidence that the complex family dynamics may take an emotional toll on both children and parents. Children whose parents have offspring by other partners tend to show more “externalizing behaviors” such as acting out and aggression, which psychologists find are related to dropping out of high school and delinquency later on. Also, parents who have children by more than one partner seem more likely to experience depression.

Implications for Child Support and Social Programs

Programs designed to alleviate child poverty may become more expensive and complicated to administer for children living in more complex family settings, she noted. Child support laws often were formulated assuming all the noncustodial parent’s children lived with the same custodial parent. The presence of multi-partner fertility complicates determining “what should be paid to whom, how much the nonresident parent is expected to pay if there are children spread across several households,” Carlson suggested.

Carlson cited evidence that the amount of child support paid declines for parents with multi-partner fertility and “mothers are less likely to get what they otherwise would have gotten if the kids were all with the same partner.”

Programs in the George W. Bush administration to reduce poverty by promoting marriage among low-income couples with children did not take into account the complexity of these families. Improving co-parenting skills may be a better intervention, Carlson suggested. Although the parents do not live together and may never marry, they would benefit from learning how to raise and support a child when they are no longer in a romantic relationship, she argued.

Magnifying Disadvantages

According to Carlson, the greater prevalence of multi-partner fertility among disadvantaged groups may reinforce and magnify the instability already found in unmarried couples with children. Resources—both time and money—are spread thin, “increasing inequality and the negative aspects of what is already a disadvantage.”

“In the long run, multi-partner fertility may diminish the capacity of families to rear and socialize children, and may increase stratification and inequality across and within generations,” she said.

Paola Scommegna is a senior writer/editor at the Population Reference Bureau.