Aggressive behavior is more common in kids who live with half- and step-siblings, a new study finds.
little girl shouting at brother
Credit: Richard Drury/Getty Images

I'm told by close relatives that I called the day my half-brother was born "the worst day of my life." Dramatic, I know, but there hadn't been many other momentous events in my 7-year-old life up until that point. (Unless you count the time I lost my favorite stuffed animal at the supermarket. I hope you're well, Puffy, wherever you are.)

And while I don't remember ever saying that, I do remember the anger I felt. A new person had been introduced into my structured, self-centered life, who now monopolized the majority of our shared father's attention, and once he gained motor skills, would start breaking all of my toys. My anger never manifested itself into violence, thankfully, but according to a new study, that may not be the case for other kids in similar situations.

Sociologists at the University of Michigan and University of Colorado-Boulder analyzed data from approximately 6,500 U.S. children and found that kids who lived with half- or step-siblings at the age of 5 were 10 percent more aggressive than kids in traditional sibling relationships. These findings, published in the journal Demography, drew on reports from each child's primary parent on how often the child displayed aggressive behavior, including temper tantrums and destruction of others' property.

The researchers took many factors into account to explain this link, but were unable to lock down a solid reason for it. They suggest that an uneven distribution of material and emotional resources among a family's children could have something to do with it, as well as parental absence, since all complex sibling structures lack at least one child's biological parent in the home.

One in six children in the U.S. today live with half- or step-siblings before starting kindergarten. The researchers plan to delve further into this topic, and while much still remains elusive, one thing is certain: They won't lack for subjects.

Gillian Nigro is an Editorial Assistant for and a very proud big sister. Follow her on Twitter and Instagram.