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.
Is Regression a Sign of Crisis — or a Form of Adaptation?
The first systematic description of what happens to firstborns when a sibling arrives comes from Judy Dunn and Carol Kendrick's longitudinal observations in the early 1980s. Following 40 British families for a year on either side of the second child's birth, they documented a familiar cluster: disrupted sleep, toilet regression, increased clinging, more demands, more oppositional behavior [1]. At the same time, they observed children who grew in independence and empathy during the same period — positioning "regression" not as a one-dimensional deterioration but as one of several possible responses [1].
The systematic review that synthesized more than 30 studies on sibling transition, published by Volling in Psychological Bulletin in 2012, reached a more cautious conclusion. Maternal responsiveness and warmth do tend to decline. But across other behavioral measures, findings are inconsistent. The review's bottom line: there is no strong evidence to support the model that the birth of a sibling constitutes a developmental crisis for most children [2]. A large-scale longitudinal study by Volling's group — following 241 families and published in Monographs of the Society for Research in Child Development in 2017 — found that the majority of families navigated the transition resiliently, and that the kind of "crisis" depicted in psychodynamic literature was not empirically observable in most of them [3].
In short: regression is not rare, but it can be called neither "abnormal" nor "universal." Whether it occurs, and how long it lasts, varies considerably from child to child. That is the current center of the evidence.
Which Behaviors, for How Long?
Stewart and colleagues' classic study following 41 families across 12 months showed that the character of the firstborn's response changes over time [4]. Immediately after the birth, imitation of the baby and oppositional behavior toward the mother are prominent. By four months, anxiety behaviors increase while imitation and opposition ease. Later still, as the younger sibling becomes mobile, direct conflict between siblings becomes the dominant theme [4]. The content of the response is not fixed; it shifts shape as the weeks pass.
On sleep, a time-series video observation study by Field and Reite (1984) — one of the few to use direct observation rather than caregiver report — documented longer sleep onset, more nighttime waking, and more nighttime crying around the time of the mother's hospitalization for childbirth [5]. The sample was small (16 children), but the observational methodology makes it worth noting.
As for recovery: Volling's 2012 review suggests that most behavioral changes peak within the first few months after the birth and then gradually resolve [2]. There is no clean "the regression ends at X months" threshold, but a reasonable overall picture is that most children return to roughly their prior baseline within six months to a year.
One point deserves particular emphasis. Whether or how much a child regresses says nothing about that child's "strength" or character. A 2021 longitudinal study by Volling and colleagues found that children's attachment security: the degree to which a child trusts their caregiver as a safe base, classified as secure, anxious, or avoidant based on Ainsworth's Strange Situation to their mothers followed four distinct trajectories across the sibling transition — and the single strongest predictor of which trajectory a child landed on was not the child's temperament, but the quality of the parents' coparenting: the degree to which two caregivers coordinate, support, and respect each other's parenting roles (collaborative parenting between partners) [6]. In other words, "whether the older child cries or doesn't cry" is less a matter of that child's makeup than of how the family as a whole is functioning.
The Quiet Work of Reassuring Attachment
Before turning to what parents can do, one framing point is worth establishing. In most cases, attachment has not "broken" — it has "wobbled." The 2021 Volling study found that while some children did show a decline in attachment security to their mother after the sibling's birth, in approximately 68% of families the decline was mild, with overall security remaining within a normal range [6].
Starting from that premise, what's needed is less "repair an injury" than "acknowledge the wobble and create more time for things to settle." Concretely, that might look like:
- Setting aside one-on-one time with the older child, however brief — ten minutes after dinner, ten minutes before bed — and protecting that slot consistently
- Not overreacting to regressive behaviors (wanting a bottle, reverting to diapers). Allow them temporarily and let the child find their own way back at their own pace
- Consciously reducing the refrain of "You're the big kid now" and increasing language that treats the child as an individual, not a role
- When the older child interrupts the baby's care, pulling them in when possible — let them hand you the wipe, hold the toy — rather than sending them away
These are not dramatic interventions. But a study by Pike and colleagues involving 101 British families found that sibling relationship quality and children's individual adjustment influence each other in both directions [7]. Small, consistent investments in the early period after the birth may shape the warmth of the sibling relationship for years afterward.
The Boundary Between SOS and Normal Regression
Most regression resolves with time. There are, however, some signals that are worth knowing as thresholds for consulting a pediatrician or child psychologist. Volling's 2012 review notes that prolonged or extreme responses should be evaluated in the context of family-level factors [2]. As rough markers, not diagnostic criteria:
- Regression that remains pronounced beyond six months — sustained disruption in sleep, toileting, or eating
- Self-injurious behavior, or repeated and intentional harm toward the younger sibling
- Broad loss of interest in smiling, playing, or other activities that were previously engaging
- Parents themselves feeling so depleted that they can no longer sustain the relationship
If any of these apply, that is not "overthinking it" — it is a reasonable occasion to speak with a family doctor or a local child-development counseling service. There is no such thing as consulting too early.
Recording as a Navigational Aid
When regression begins, parental memory tends to get colored entirely by "things have gotten worse." In reality, new capabilities are often appearing alongside the things that have fallen back. Using an app like Memori to scroll back through the weeks before and after the birth sometimes makes small growth that runs parallel to the regression visible. If you are keeping a daily log, consider noting not just the regressive moments but also "one thing that went normally today." The difference in what you see when you read back later is real.
Summary
Some firstborns regress when a sibling arrives; some do not. Duration varies. Most return to something close to their prior baseline within six months to a year, and the quality of collaborative parenting between parents is a better predictor of that recovery than any trait of the child's [2,3,6]. Regression in itself is not abnormal. Prolonged regression, extreme behaviors, self-injury, or repeated aggression toward the sibling are signals worth taking to a professional.
"Becoming a big brother or sister" is probably not something that happens in a moment when a new title arrives. It is something that happens gradually, through wobbles and recoveries, over a longer stretch of time.
References
- Dunn J, Kendrick C. Siblings: Love, Envy, and Understanding. Cambridge, MA: Harvard University Press; 1982.
- Volling BL. Family transitions following the birth of a sibling: an empirical review of changes in the firstborn's adjustment. Psychol Bull. 2012;138(3):497–528. doi:10.1037/a0026921. PMID: 22289107.
- Volling BL, Gonzalez R, Oh W, et al. Developmental trajectories of children's adjustment across the transition to siblinghood: pre-birth and sibling outcomes at year one. Monogr Soc Res Child Dev. 2017;82(3):7–48. doi:10.1111/mono.12309.
- Stewart RB Jr, Mobley LA, Van Tuyl SS, Salvador MA. The firstborn's adjustment to the birth of a sibling: a longitudinal assessment. Child Dev. 1987;58(2):341–355. PMID: 3829782.
- Field T, Reite M. Children's responses to separation from mother during the birth of another child. Child Dev. 1984;55(4):1308–1316. doi:10.2307/1129999.
- Volling BL, Steinberg EJ, Kuo PX. Is one secure attachment relationship better than none? Changes in children's attachment security to mother and father after the birth of a sibling. Dev Psychopathol. 2023;35(1):137–151. doi:10.1017/S0954579421001152. PMID: 34903310.
- Pike A, Coldwell J, Dunn JF. Sibling relationships in early/middle childhood: links with individual adjustment. J Fam Psychol. 2005;19(4):523–532. doi:10.1037/0893-3200.19.4.523.