Beginning in the 1960s, rising divorce rates in this country produced an increasing number of blended families; more recently, young people have been putting off marriage, leading to an unprecedented number of children born outside of marriage: Today, 41 percent of all children are born to unmarried parents. Among disadvantaged groups, the proportion is even higher.

For researchers, this is relatively new ground. Previous studies have suggested that marriage and biological ties are associated with greater investment in children. But does that hold true for the full range of family types?

The answer seems to be, not always. In a new study, Marcia Carlson and Lawrence Berger, both of the University of Wisconsin-Madison, examine how parental investment in time and money varies across six different family types: biological parents married and cohabiting, mother and step-father (known to researchers as a “social father”) married and cohabiting, and biological parents dating and not dating. Using data from the Fragile Families and Child Wellbeing Study, Carlson and Berger looked at the connections between these family type and two factors: income and frequency of parent-child activities like reading, watching television, and going on outings. The results were published in the June 2013 issue of Social Service Review.

What they found was at once predictable and surprising. Children living with married biological parents benefitted from greater income and more frequent activities than all other family types—with one exception. Children living with their married mother and a social father received more frequent parental attention than in any other family type, including married biological parents. They received slightly more attention from their social fathers than did children living with their biological fathers—and they also had a small amount of interaction with their absent biological fathers.

Although this result surprised the researchers, Carlson says it’s hard to draw too many conclusions because the sample was small—only about two percent of families—and because social fathers, even when married, may not develop enduring bonds with their spouse’s children. “If and when the relationship ends, those guys have very little reason to continue a relationship with the kids,” she notes.

Another unexpected finding, Carlson says, is how much a difference marital status makes for social fathers. On average, married social fathers are far more involved with their step-children than social fathers who are merely cohabiting. For biological fathers, on the other hand, this difference matters less than simple co-residence. In other words, they spend less time with their children only when they aren’t living with the mother.

Still, the causes of these different patterns remain obscure, Carlson says. Do different family structures encourage different kinds of parenting—or do the differences reside in the people who end up in different family types?

Carlson says her study raises many questions for further research, including one that she and Berger are now hoping to answer: What consequences do differences in parental attention have for children’s ultimate well-being?

Carlson, Marcia J. and Lawrence M. Berger. “What Kids Get from Parents: Packages of Parental Involvement across Complex Family Forms.” Social Service Review 87 (2): 213 – 249.