Children living with half/step-siblings show more aggression
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Young children living with half-siblings or step-siblings show more aggressive behavior

By Paula Fomby and , | January 2016 

One in six young US children live in complex sibling relationships, which are associated with heightened aggressive behavior – even if both parents live together.

Young children raised with half-siblings or step-siblings behave more aggressively, even when they themselves are living with both their own parents. That’s a key finding from research that shows that one in six young US preschool children are growing up in complex sibling relationships – far more than we previously knew.

Children who lived with step- or half-siblings at age 4 had considerably higher scores for aggressive behavior when they entered school a year later. Based on reports by their mothers, their scores were about 10 percent higher than those reported for otherwise similar children. This increase occurred regardless of whether children lived with both of their biological parents, a single parent, or one biological parent and a stepparent.

“Only a minority (16 per cent) of children living with half-siblings or step-siblings lived with their mother and a stepfather at age 4. More than half lived with both of their own parents, while a third were living with their unpartnered mother.”

Sibling relationships as a factor in family organization have received relatively little attention from researchers. Most recent studies of complex families have focused instead on how children’s welfare is influenced by relationships between parents or relationships between parents and children.

Among young children, aggressive behavior includes, for example, hitting people, anger, and destruction of property. The heightened levels of aggressive behavior we found at age 5 suggest a lack of school readiness. Aggressive behavior in young children also predicts increased risky behavior during adolescence and lower educational attainment in early adulthood. So researchers need to discover why living with a half-sibling or step-sibling carries this increased risk of aggressive behavior in early childhood.

The majority live with both parents

Among children who experience complex sibling relationships, family circumstances vary. We often think that children who live with a half-sibling or step-sibling would also be living with a stepparent. In reality, the majority (52 percent) are living with both their own parents and with half-siblings born in their parents’ earlier relationships. A minority (16 per cent) of children living with half-siblings or step-siblings reside with their mother and stepfather; about a third (32.7 per cent) live with their unpartnered mother. (Our analysis excluded the small percentage of children who did not live with their biological or adoptive mother.)

Why is aggressive behaviour more frequent?

Why might complex sibling relationships be associated with heightened aggressive behavior, regardless of whether a child is living with a single parent, both parents, or a parent and stepparent? Our study of a nationally representative sample of about 6,500 US children allowed us to rule out possible explanations such as material resources available to the family, background characteristics and current wellbeing of mothers, and mothers’ parenting style. Future research might focus on other possible explanations, including at what point in their lives step- and half-siblings enter children’s households. It might also examine whether material and emotional resources are distributed evenly to each child in a family.

Could parental absence be a factor?

Across family structures, all children living with a step- or half-sibling have one thing in common: at least one child in the household has an absent biological parent, either living elsewhere or no longer alive. Prior work has shown that having an absent parent, and particularly an absent father, is associated with a higher risk of aggressive behavior in younger children. In future research, we will consider how the behavior of children with an absent parent might affect other children in the household whose parents are both present.

During the past 40 years, family organization in the United States has become increasingly complex and varied. Today’s young children frequently experience family as a set of biologically and socially based relationships within and across households. We would encourage further exploration into how and why sibling relationships – and particularly step- and half-sibling relationships – may be uniquely associated with young children’s development and wellbeing.


 Fomby P, Goode J & Mollborn S (2015), Family complexity, siblings, and children’s aggressive behavior, Demography, 53.1

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