Preparing a toddler for a new sibling: what sibling-adjustment research says

There is a particular look on a toddler's face when the new baby comes home: equal parts fascination and dawning suspicion. The research on what happens next is some of the warmest and most human in developmental psychology — and it is far more reassuring than the folk wisdom about jealous firstborns suggests.

The Cambridge sibling studies: what actually changes

The foundational work is Judy Dunn and Carol Kendrick's longitudinal research at Cambridge, which observed families at home before and after a second child's birth. Their 1980 study of 41 families documented precisely what shifts: after the baby arrived, mothers had measurably less time for attention and play with the firstborn, confrontations increased, and the firstborn increasingly had to initiate interactions themselves.¹

A companion study using mothers' detailed reports found that firstborns' reactions were wide-ranging — more demanding and tearful behaviour and toileting lapses in some children, but also, strikingly, real interest in and affection for the baby, and new independence.² Dunn and Kendrick's larger message, developed in their classic book Siblings: Love, Envy and Understanding, was that the sibling relationship is emotionally rich from the very start — the jealousy and the love are not alternatives; they coexist.

Not a crisis: what the modern review found

For decades, clinicians talked about the second baby as a "developmental crisis" for the firstborn. The most comprehensive modern review — Brenda Volling's 2012 synthesis of 30 studies in Psychological Bulletin — tested that idea directly, and rejected it. The evidence did not support a crisis model: while affection and responsiveness toward mothers dipped, the findings on sleep problems, anxiety, aggression and regression were mixed, and individual differences were enormous. For some children the transition is disruptive, for some it is an occasion for developmental leaps, and for many it passes with no dramatic change at all.³

Later longitudinal work by Volling's team, following families across the first year after the birth, reinforced the point: most firstborns in community samples do not develop clinically concerning behaviour problems after a sibling arrives.⁴ The transition is a genuine adjustment — but your toddler is built for it.

Regression is normal (and temporary)

The most alarming-looking part of the adjustment is usually regression: the confidently toilet-trained child having accidents, the independent sleeper wanting the cot back, the fluent talker adopting a baby voice. Dunn and Kendrick's mothers reported exactly these behaviours in the weeks after the birth² — and the modern review found regression to be neither universal nor, where it occurs, a marker of lasting problems.³

The logic from a toddler's perspective is impeccable: the small person who cannot walk, talk or use the toilet is receiving round-the-clock devotion. Acting like a baby is a hypothesis about how attention works — not manipulation, and not a step backwards in development. The evidence-consistent response is low-key: meet the need behind the behaviour (closeness, reassurance), avoid shaming, and let the regression fade on its own schedule — usually weeks, not months. Punishing it tends to entrench it.⁶

Before the birth: preparation that works

Australian guidance recommends starting preparation around three to four months before the due date — early enough to normalise the idea, late enough that a toddler's sense of time can hold it.⁵ The recurring themes in the guidance map neatly onto what the research says drives adjustment:

After the birth: involvement over protection

Dunn and Kendrick noticed something in their home observations that has shaped advice ever since: the family conversations that went best were ones where the firstborn was drawn into the baby's care and talked with about the baby as a person — "look, she's watching you" — rather than managed around the baby's needs.¹ Involvement gives the firstborn a role; protection gives them a rival.

• Offer real jobs: fetching nappies, choosing the bath toy, singing to the baby.
• Narrate the baby's interest in them: "He always calms down when he hears your voice."
• Accept ambivalence gracefully. "You wish the baby would go back" is a feeling to name, not a scandal to correct. Naming feelings is a skill worth building in general — our Emotion Decoder and our piece on why transitions are hard for toddlers both dig into why.

Protecting one-on-one time

The single most consistent thread across the literature and the guidance is attention. What Dunn and Kendrick measured — the drop in maternal attention and play after the birth¹ — is exactly what Australian guidance targets with its most practical advice: deliberately protect small pockets of one-on-one time with your firstborn, especially in the early months.⁶

It does not need to be long. Ten minutes of undivided attention — during the baby's nap, at bedtime, on a solo walk to the shops — reliably does more for a toddler's sense of security than hours of divided attention. Bedtime is a particularly good anchor: keeping the firstborn's familiar routine intact (same books, same songs, same order) preserves an island of predictability while everything else shifts. If sleep is wobbling anyway, our piece on preparing for big transitions covers why familiar rituals matter so much in seasons of change.

When to talk to a professional

A final note on timing your expectations: the adjustment is not a single event but a season. Many families find the first fortnight deceptively smooth — visitors, novelty, adrenaline — with the harder weeks arriving around the one-to-two-month mark, when the excitement has faded and the baby's demands have not. If that is where you are now, it is not a sign that something has gone wrong; it is the ordinary middle of the curve the longitudinal studies describe, and it bends back up.⁴

The Cambridge studies left one more finding worth carrying with you: within weeks, most firstborns showed genuine affection for their sibling — comforting them, entertaining them, narrating the world to them.² The rivalry is real, but it is half the story. The other half is the beginning of one of the longest relationships of your children's lives.