Kids with step- or half-siblings are more likely to behave aggressively than those who don't have this added complexity in their family structure, researchers at the University of Michigan report in the journal Demography. They studied about 6,500 children and their families from across the country in the Early Childhood Longitudinal Study, asking the primary parent (most often the biological mother) of each young child (under the age of 5) about frequency of things such as temper tantrums, physical aggression, shows of anger, and destruction of personal property. It turns out that one in six kids in the US live with step- or half-siblings before they enter kindergarten. And those kids with more complex family structures had aggression scores about 10% higher than peers who didn't live with step- or half-siblings.
While this link does not establish causation, the findings "add nuance to the prior scientific conversation on family and development, which focused on parental-union status, not siblings," reports Quartz. Study author and sociologist Paula Fomby does point out that most young kids in "complex sibling relationships" don't actually live with a stepparent, but rather their own biological parents or a single mother. But what all kids who live with step- or half-siblings have in common, she says, is that "at least one child in the household has an absent biological parent, either living elsewhere or no longer living," which is what she plans to study next. (Check out how many marriages involving a stepfather don't last.)