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When a Sibling Arrives: Reading the Firstborn's Regression as Development

By Leon MoriguchiPublished May 13, 2026Updated May 18, 20267 min read日本語版あり
Audience
Parents of firstborn children aged 2–3

TL;DR

  • ·A large empirical review finds no strong evidence that a sibling's birth constitutes a developmental crisis for most firstborns; regression is common but not universal, and most children return to baseline within six months to a year
  • ·How the firstborn fares depends not only on the child's own temperament but also on family factors such as the quality of collaborative parenting between partners (coparenting)
  • ·Regression is not the only thing happening — new capabilities often develop alongside it, and logging both gives a truer developmental picture than focusing on setbacks alone

Contents

  1. Lead
  2. Is Regression a Sign of Crisis — or a Form of Adaptation?
  3. Which Behaviors, for How Long?
  4. The Quiet Work of Reassuring Attachment
  5. The Boundary Between SOS and Normal Regression
  6. Recording as a Navigational Aid
  7. Summary
  8. References

Lead

Since the baby was born, the two-and-a-half-year-old has been asking for a bottle. The toilet is not making it in time anymore. "Carry me" has become the refrain of every hour. Things that used to happen without a second thought have stopped happening.

When parents see this "going backward," two feelings tend to arrive together. The first is sympathy: the older one must feel left out. The second is anxiety: is this a sign of something wrong? Most parenting books are written in the language of the first feeling; they are less good at articulating the second with any precision.

This article uses the language of empirical research, not impression, to lay out what actually happens to a firstborn when a younger sibling arrives. How common is regression? How long does it last? And where is the line where "this warrants a conversation with someone" begins? The goal is to give parents at least a rough way to distinguish developmental regression from regression as an SOS.

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