Today, about 1 in 6 US children live with step- or half-siblings at age 4. Paula Fomby and colleagues studied the frequency of aggressive behaviors – tantrums, physical aggression, property destruction, etc. – among kindergarten students. They found that children living with step- or half-siblings had more of these behaviors than their peers whose parents had the same union status but whose households didn’t include step/half-siblings. Fomby says potential explanations for this difference include an uneven distribution of materials and emotional resources across children in these households, as well as the absence of one parent.

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