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The Spillover Effect When One Sibling is in a Study

September 27, 2023

Usually, when you enroll participants in a pediatric research study, you only enroll one child in the family. But are siblings of that child also affected by the study?

Joshua Jeong, ScD and colleagues at Harvard T.H. Chan School of Public Health looked at this question, specifically for an intervention to positively change parenting and feeding practices. This week, Pediatrics is early releasing their results in an article entitled “Effects of a Parenting and Nutrition Intervention on Siblings: A Cluster-RCT” (10.1542/peds.2023-061383). 

The specific intervention took place in Tanzania and provided mothers and fathers with information about responsive parenting, infant and toddler feeding, and how to create positive relationships with each other.

Perhaps it’s not a surprise that the authors found that the siblings of the “index child” (i.e., the child whose outcomes were measured in the study) also benefited! Compared to siblings in the control group, the intervention siblings had:

  • Better expressive language
  • Better dietary intake
  • Fewer internalizing behavioral problems – meaning, behaviors directed inwardly towards oneself. Examples would be social withdrawal, depressive symptoms, and anxiety symptoms.

What might cause these spillover effects? The authors speculate that a combination of factors may be at play. First, parents will likely apply what they’ve learned in the intervention with all of their children. Second, the “index” children may influence their siblings by modeling different behaviors or through more positive interactions.

Even though this study took place in Tanzania, I think that the lessons learned here are likely much more far-reaching. This study reminds us that, when we educate and train parents, the effects can be positive for all of the children in the family.

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