Summary: Does being the “firstborn” give you a biological head start? A large-scale Japanese study suggests that the answer is yes, and it starts much earlier than previously thought. By comparing over 2,000 sibling pairs within the same families, researchers found that second-born children score lower on developmental markers as early as 6 months of age.
While some of these gaps—like communication and problem-solving—narrow by the 12-month mark, others persist. The study points to “resource dilution” as a key factor, as parents naturally have less time and energy to engage with a second child compared to their first.
Key Facts
- 6-Month Milestone: Second-born infants scored lower across all developmental domains at 6 months, including communication, motor skills, and personal-social skills.
- 12-Month Catch-Up: By one year, the gap in communication and problem-solving narrowed significantly, likely because later-born children benefit from interacting with their older siblings (the “confluence model”).
- Persistent Gaps: Differences in fine motor skills and personal-social development remained evident at 12 months.
- Resource Dilution: The study found a direct correlation between lower developmental scores and reduced parental engagement (reading, playing, and outdoor time) for second-born children.
- Sibling Comparison: Because the study compared siblings within the same family, it effectively ruled out stable factors like socioeconomic background or general parenting style.
Source: University of Toyama
Birth order has long been linked to differences in cognition, with firstborn children often outperforming their later-born siblings. Parental engagement and interaction have been suggested as potential influences on this difference.
However, only a few studies have compared siblings within the same family, particularly in the first year of life, making it difficult to fully separate birth order from broader family background factors. Such within-family comparisons may help address this gap.
The within-family comparison was made possible by the Japan Environment and Children’s Study (JECS). JECS is a nationwide birth cohort study supported by the Ministry of the Environment, Japan, designed to investigate environmental factors affecting children’s health and development.
Women who became pregnant again during the recruitment period were able to re-enroll, allowing researchers to identify sibling pairs within the same family. In total, approximately 5,000 sibling pairs have been registered in the cohort, of which 2,117 firstborn–second-born pairs were included in the present analysis.
Using data from JECS, the study compared neurodevelopmental outcomes between firstborn and second-born children. It was led by Assistant Professor Akiko Tsuchida from the University of Toyama, Japan, together with co-authors from the University of Toyama, Aomori University of Health and Welfare, and Gunma University.
Their study was published in the Jama Network Open journal on March 6, 2026.
“The research idea was sparked by a news article that noted many players on Japan’s national football team are later-born children, with very few firstborns represented,” said Dr. Tsuchida.
“That observation made me wonder how siblings—who share much of the same genetic background—can nevertheless show systematic differences in outcomes. As I reviewed the literature, I found consistent evidence suggesting that firstborns often have advantages in cognitive and socioeconomic outcomes. Theoretical explanations such as parental investment and the confluence model have been proposed, but I wanted to understand how early in life such differences might first emerge.”
The study was based on 2,117 pairs of firstborn and second-born singleton siblings. The Ages & Stages Questionnaire®, Third Edition (ASQ®-3), a popular parent-completed developmental screening tool, was used to evaluate all children at 6 and 12 months. The researchers employed a mother fixed-effects approach, which compares siblings within the same family.
This approach accounts for shared, stable family characteristics, such as parenting style and socioeconomic background. The researchers also measured parental engagement using a 5-item composite score, capturing how often caregivers played with, read to, and took their infants outdoors.
At 6 months, second-born infants scored lower than their firstborn siblings across all ASQ®-3 domains, which included communication skills, gross motor skills, fine motor skills, problem-solving skills, and personal-social skills.
The gap varied across domains, ranging from a 1.9-point difference in communication to a 13.8-point difference in personal-social development. By 12 months, differences remained for fine motor and personal-social skills, while those in communication and problem-solving were smaller and no longer statistically significant.
These reduced differences observed at 12 months may be partly explained by the ‘confluence model,’ which suggests that a child’s intellectual development is shaped by the average intellectual environment of their home. Later-born infants may benefit from observing and interacting with older siblings as they grow, potentially offsetting reduced parental resources.
An interesting pattern also emerged in the parental engagement, with second-born children having lower engagement scores than firstborns. These parallel reductions in both developmental scores and parental engagement are consistent with the ‘resource dilution model,’ which proposes that as family size grows, the time and attention available per child may decrease. Reduced parental engagement may be related to early developmental differences.
“Our findings suggest that parental engagement may be one piece of the complex processes underlying early neurodevelopmental differences,” Dr. Tsuchida concluded.
“Understanding these dynamics may help inform efforts to support children’s development, regardless of birth order.”
Funding information
The Japan Environment and Children’s Study (JECS) was funded by the Ministry of the Environment, Japan.
Key Questions Answered:
A: This study provides concrete evidence that it’s real and starts in infancy. At 6 months old, second-born babies often lag behind where their older siblings were at the same age. However, it’s not necessarily a permanent disadvantage; the gap in several key areas often closes by the time the child turns one.
A: It largely comes down to “resource dilution.” When the first child is born, they get 100% of the parents’ attention. When the second arrives, that time is split. The study showed that parents spend less time reading to and playing with their second child, which directly impacts early developmental scores.
A: Yes! The study suggests that while they get less “one-on-one” parent time, second-borns benefit from having a sibling to observe and mimic. This is likely why they catch up so quickly in areas like communication and problem-solving by their first birthday.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- Journal paper reviewed in full.
- Additional context added by our staff.
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
Birth order differences in cognitive and developmental outcomes are well documented, with firstborn children typically outperforming their later-born siblings. Several explanations have been proposed, including parental learning and changes in family circumstances.
In infancy, when parental engagement plays a central role, resource dilution has been suggested as a contributing mechanism. However, only a few studies have compared siblings within the same family, particularly in the first year of life, to separate birth-order factors from stable family factors.
We, therefore, examined whether firstborn and second-born children differ in neurodevelopmental outcomes during infancy using a mother fixed-effects design.
