Summary: Does being the firstborn give a biological and developmental head start? A large-scale Japanese cohort study indicates the answer is yes — and that differences appear much earlier than previously recognized. By comparing more than 2,000 sibling pairs within the same families, researchers found that second-born infants scored lower on standard developmental measures as early as six months of age.
Some of these early gaps — notably in communication and problem-solving — reduce substantially by 12 months, while other differences, such as fine motor and personal-social skills, remain evident. The investigators point to reduced parental engagement with later-born children, often described as “resource dilution,” as a likely contributing factor.
Key Facts
- 6-month milestone: At six months, second-born infants scored lower than their older siblings across all measured developmental domains, including communication, gross and fine motor skills, problem-solving, and social interaction.
- 12-month catch-up: By one year, gaps in communication and problem-solving had narrowed and were no longer statistically significant, likely reflecting early learning from older siblings.
- Persistent differences: Fine motor and personal-social score differences persisted at 12 months, indicating some developmental domains remain sensitive to early variation in caregiving.
- Parental engagement: The study linked lower developmental scores with reduced time spent reading, playing, and going outdoors with second-born infants — supporting the resource dilution explanation.
- Within-family comparison: By comparing siblings in the same household, researchers minimized confounding from stable family characteristics such as socioeconomic status or parenting style.
Source: University of Toyama
Birth order has long been associated with differences in cognitive and developmental outcomes, with firstborn children often showing early advantages. Parental engagement and interaction have been proposed as key influences on these differences.
Few previous studies have had the scope to compare siblings within the same family during the first year of life, a period when parental interaction strongly shapes early neurodevelopment. This study uses within-family comparisons to separate birth-order effects from stable background factors.
The sibling comparisons were possible through the Japan Environment and Children’s Study (JECS), a nationwide birth cohort sponsored by the Ministry of the Environment, Japan. Women who became pregnant again during the recruitment window re-enrolled, allowing investigators to identify sibling pairs. From roughly 5,000 registered sibling pairs, the present analysis included 2,117 firstborn–second-born singleton pairs.
Using JECS data, the research team led by Assistant Professor Akiko Tsuchida at the University of Toyama compared neurodevelopmental outcomes between first- and second-born children. Co-authors were affiliated with the University of Toyama, Aomori University of Health and Welfare, and Gunma University. The full study was published in JAMA Network Open in March 2026.
The investigators were motivated by observations that birth order correlates with later-life outcomes, and by an interest in identifying how early such differences first appear. They used the Ages & Stages Questionnaire® (ASQ-3), a parent-completed screening instrument, to assess development at six and 12 months. A mother fixed-effects analytic design compared siblings within the same family, controlling for stable household characteristics, while a five-item parental engagement score measured frequency of playing, reading, and outdoor activities.
At six months, second-born infants scored lower than their firstborn siblings in every ASQ-3 domain. The magnitude of differences varied by domain — from around a two-point gap in communication to a larger gap in personal-social development. By 12 months, the differences in communication and problem-solving had diminished and were no longer statistically significant, but disparities in fine motor coordination and personal-social measures persisted.
The reduction of some gaps by the one-year mark is consistent with the confluence model, which posits that a child’s cognitive environment reflects the average intellectual and social influences in the household. Later-born infants may benefit from observing and interacting with older siblings, accelerating their development in certain areas despite receiving less exclusive parental attention.
At the same time, the study documented lower parental engagement scores for second-born infants. These parallel declines in caregiver interaction and developmental measures support the resource dilution model, where caregiving time and investment per child decrease as family size grows. Reduced one-on-one interaction during infancy may contribute to early developmental differences between siblings.
“Our findings indicate that parental engagement is one factor in the complex processes shaping early neurodevelopmental differences,” Dr. Tsuchida said. “Recognizing how birth order and caregiving interact could help inform strategies to support children’s development, regardless of their birth position.”
Funding information
The Japan Environment and Children’s Study (JECS) was funded by the Ministry of the Environment, Japan.
Key Questions Answered:
A: This large sibling-comparison study supports the existence of an early firstborn advantage. At six months, second-born infants frequently lag behind where their older siblings were at the same age. However, the difference is not uniformly permanent; several gaps, particularly in communication and problem-solving, shrink by 12 months.
A: The study highlights resource dilution — less one-on-one parental time and engagement for later-born children — as a key mechanism. Parents generally spend less time reading to, playing with, and taking second-born infants outdoors, and those interactions are closely linked to early developmental scores.
A: Yes. While second-borns may receive less exclusive parental attention, they often gain from sibling interaction and modeling, which can accelerate gains in communication and problem-solving and contribute to a rapid catch-up in some domains.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- The journal paper was reviewed in full.
- Additional context was added by staff to clarify study methods and implications.
About this neurodevelopment research news
Author: Yumiko Kato
Source: University of Toyama
Contact: Yumiko Kato – University of Toyama
Image: The image is credited to Neuroscience News
Original Research: Open access. “Birth Order Differences in First-Year Neurodevelopment” by Akiko Tsuchida, Kenta Matsumura, Haruka Kasamatsu, Tomomi Tanaka, Kei Hamazaki, and Hidekuni Inadera; for the Japan Environment and Children’s Study Group. JAMA Network Open. DOI: 10.1001/jamanetworkopen.2026.1265
Abstract
Birth Order Differences in First-Year Neurodevelopment
Differences by birth order in cognitive and developmental outcomes are well documented, with firstborn children frequently showing early advantages. Proposed explanations include changes in parental investment and shifts in the home intellectual environment. During infancy, when parental engagement plays a central role, resource dilution is a leading candidate explanation. However, few studies have used within-family comparisons in the first year to separate birth-order effects from stable household factors. This analysis applied a mother fixed-effects design to examine neurodevelopmental differences between first- and second-born infants during the first year of life.