When we think about how families shape children’s well-being, we tend to concentrate on the relationship between parents and children. Is a child living with his biological parents, and are they married? Or is she living with one biological parent plus a step-parent? If he lives with an unmarried parent, is he exposed to a succession of surrogate parents in the form of that parent’s romantic partners? We know that these factors matter for children’s well-being, and since there’s a relatively large amount of relevant data on them available, it’s no surprise that such questions dominate discussions of the family today.

Yet another, much less studied aspect of children’s family lives also influences their chances of well-being and success: their relationships to their siblings. That is, whether a child has step-siblings, half-siblings, or only full biological siblings can affect her outcomes independent of her relationship to her parents. The available data—and consequently the research—in this area are limited, but the existing evidence suggests that even for children living with both their biological parents, the presence of half- or step-siblings in the household is associated with negative outcomes, including greater depression and delinquency and worse academic performance.

In a new working paper for the National Center for Family & Marriage Research, Susan L. Brown, Wendy D. Manning, and Bart Stykes extend this line of research by investigating how both family structure (parents’ relationship to each other and to their children) and family complexity (siblings’ relationships to one another) are related to children’s economic well-being. Rather than simply examining whether or not a child lived in poverty, the researchers looked at the family’s income to needs ratio and the share of family income derived from work earnings (as opposed to unearned income, like public transfers). Their sample, drawn from a Census survey, included roughly 13,000 children under eighteen, 9 percent of whom lived in complex families, in a range of single- and two-parent family structures, and they controlled for factors such as children’s race and age, maternal age, parental education, and the number of children in the family.

Brown and her coauthors found that family complexity is linked to worse economic well-being for children regardless of family structure. This link remained evident even after controlling for the aforementioned factors when in came to families’ income to needs ratio. Family complexity was also negatively related to the share of income derived from earnings, but that link became nonsignificant when controls were included, a fact that the researchers suggest may be because “the range to be explained was relatively narrow to begin with.”

Since the data source did not allow for examining how families’ structure, complexity, and economic status changed over time, Brown, Manning, and Stykes note it is not possible to draw causal conclusions from their findings. (On that question, further research is needed, as the scholarly refrain goes.) Nevertheless, the results of their investigation, coupled with prior research on other domains of child well-being, indicate that family complexity should certainly be measured and studied along with family structure as a factor relevant to children’s flourishing.