CHALLENGE: 4-year-old hits/pushes his 18-month-old sister when she touches his toys, even with duplicates available. Started when sister became mobile/walking. 1. DEVELOPMENTAL LENS At 4, kids have some impulse control but it's still fragile — especially under threat to something they value. This is common and age-typical, not a red flag on its own. What's notable is the trigger: it's not really about the toy, it's about his sister's mobility. Before she walked, she couldn't invade his space. Now she can, and his world just got less predictable. 2. THE COS READ This reads as an exploration-related disruption, not a comfort-seeking one. His "secure base" — his space, his stuff, his sense of being the one who gets to explore freely — just got a new, unpredictable variable in it. The hitting is likely less about the sister and more a signal that his sense of safe territory has shifted and he doesn't have the words or strategy for it yet. 3. WHAT'S LIKELY DRIVING IT Most likely: displacement — normal sibling-rivalry-adjacent frustration at losing exclusive access to his space, compounded by not yet being able to predict or control when she'll approach. Less likely: attachment insecurity — nothing here suggests he's anxious about losing your attention, just annoyed at losing control of his stuff. 4. HOW TO RESPOND IN THE MOMENT Name the feeling before the correction, then hold the boundary: "You're mad she took your block. I won't let you hit — hitting hurts. Let's tell her 'no' with your words." Avoid long lectures in the moment — his capacity to absorb reasoning is lowest right when he's dysregulated. 5. WHAT TO BUILD OVER TIME Give him a predictable "safe zone" — a shelf, bin, or space that's his alone, where sister isn't allowed. This rebuilds a sense of control over his environment without requiring him to share everything. Also build in 1:1 time with you that isn't about the sister at all, so his "base" still feels intact. 6. WHEN TO SEEK SUPPORT Not needed yet — this is typical. Flag for a pediatrician or therapist if: aggression escalates in intensity/frequency despite consistent response, extends beyond his sister to other kids, or comes with signs of high anxiety elsewhere.