Does Living with Step- or Half-Siblings Make Children More Aggressive?

A recent study found that children who live with step- or half-siblings are more likely to behave aggressively than kids who don't. We got in touch with the lead researcher to figure out why.

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This article originally appeared on VICE US

A lot of people in America get divorced, so it's no surprise that there are a lot of "complex families" in this country. According to a new study, roughly one in six children live with a step- or half-sibling before kindergarten. While pop culture likes to portray or even stigmatize step-kids as bullies (see Sixteen Candles, Cruel Intentions, etc.), recent research may suggest that the silver screen isn't just perpetuating stereotypes.

In an article published in the February edition of journal Demography, sociologists found that children who live with step- or half-siblings are more likely to behave aggressively than kids who don't. Led by sociologist Paula Fomby from the University of Michigan Institute for Social Research and supported by a grant from the National Science Foundation, the study utilized data from a large project called the Early Childhood Longitudinal Study, including observational research conducted on approximately 6,500 kids at multiple stages before entering primary school. Fomby and her team took the information with the aim of analyzing whether living with complex siblings at age four could predict behavior at age five. And they found that, on average, they are more aggressive, with step- or half-siblings scoring ten percent higher than their counterparts in personality tests measuring aggressive behavior.

Whereas past studies on complex families have focused on children in relation to their biological or step parents, this research shifted the lens to focus on sibling relationships. But while Fomby and the other sociologist noticed a clear correlation, the reasons as to why young kids with complex siblings behave more aggressively remained elusive. We spoke with the sociologist to discuss her findings and ask if living with step-siblings at a pivotal age can actually predict how kids will act in the future.

For more on families, watch our doc on America's lucrative divorce industry:

VICE: What was your original intention with this study?
Paula Fomby: My colleagues and I have spent a lot of time looking at how family structure is associated with children's behavior, and particularly changes in family structure. We spent many years looking at how parents' divorce or remarriage affects children. We weren't able to explain a lot about the effects we observed, so I thought, Let's look at this from another angle. I want to think about other kids in a child's family and how the composition changes in terms of siblings as children go through family structure shifts. It's bringing the lens of thinking about sibling relationships to this persistent question about why family structure change is associated with children's behavior.

So what were you looking for by studying sibling relations instead of the parent-child relations?
We wanted to get a sense of whether there was an independent effect of living with half- and step-siblings, apart from the effect of your parents' relationship status. That was the really striking thing that came out of this paper: there was this very independent effect, sort of consistent in its magnitude, regardless of whether they lived with their own two [biological] parents, with one parent and one stepparent, or with a single parent. That effect of living with a half- or step-sibling was consistently the same.

What were your findings?
We found that living with a half- or step-sibling when you're four years old seems to increase your expressions of aggressive behavior by about one point, compared with if you don't live with step- or half-siblings. So it was consistently and statistically significant, [but] it doesn't push kinds into extreme behavior. It doesn't mean that kids are clinically anti-social or anything like that. They are fairly small findings and I don't mean to imply that we have some kind of epidemic on our hands, but it's a consistent pattern.

Will you explain the scale you were using to measure aggressive behavior?
We were looking at a 28-point scale for seven different kind of behaviors that constitute aggressive behavior for kids that are going into kindergarten. Those seven items included things concerning children's physically aggressive behavior, anger, impulsivity, and destruction of property. Parents were asked whether they behaved in these behaviors "never," "rarely," "sometimes," "often," or "very often." Then we coded that on a scale from zero to four, so the possible range is 28. There were seven items and theoretically they could have been doing all of them very often. The average score was about nine, so if a child's score on average is about nine, then a one-point increase [found in children with complex siblings] would be about a ten percent increase over the baseline score.

What is it about living with step-siblings might lead a child to exhibit these aggressive behaviors?
The short answer is that our research couldn't answer this question. We considered three explanations: family resources like money, time, and emotional energy to invest in kids; parenting style; and things about a parent's background that might have led both to their complex family formation and their children's behavior problems. Taking these things into account didn't change the relationship between complex sibship and children's externalizing behavior at the time they enter kindergarten.

What other areas within this topic would you like to explore in the future based on your current findings?
There are three avenues. One is try to look directly at sibling relationship quality between step and half-siblings and full-siblings. Another is to look at where the resources within families are distributed differently, depending on the relationship between those children. And then the third is to get more information about non-resident parents—like divorced dads who the kids visit on weekends—and to think about how they are interacting with their children in the household, and the spillover effect of that dynamic on sibling relationship.

Do you have any insight into if these aggressive behaviors would translate into adulthood?
[In our research] we looked at whether living with complex siblings when they are four years old predicted their behavior when they are five, just when they are going into kindergarten. There is other research that have found relationships [between] complex siblings [and] delinquent behavior in adolescents. There is evidence that at different life stages there is a consistent association with this kind of family complexity, as well as risk taking behavior and aggressive behavior.

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