Firstborns show early developmental advantage over siblings

Birth order differences in cognition and development present in the first year of life among siblings, favoring firstborn children, according to a recent study published in JAMA Network Open.¹

A large body of evidence has supported improved performance in cognitive and developmental outcomes among firstborn children compared with their later-born siblings, with potential sources of these differences including parental learning and changes in family circumstances. Experts have also hypothesized resource dilution in infancy as a contributing factor.

“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,” wrote investigators.

Comparing siblings: a within-family design

The cohort study was conducted to assess differences in neurodevelopmental outcomes between firstborn and second-born children during infancy. These individuals were compared using a within-family design, and all were singleton children enrolled in the Japan Environment and Children’s Study.

Siblings were identified through reenrollment in the trial among participants who became pregnant twice during the study period. The Ages & Stages Questionnaires, Third Edition (ASQ-3) was used to evaluate the neurodevelopment of infants at 6 and 12 months after birth.

A 5-item composite score was used to measure parental engagement. Scores ranged from 0 to 18, and greater scores indicated increased parental engagement. This score assessed parent-child play, outdoor activities, and reading aloud, with mother fixed-effects linear models used to estimate birth order differences.

Early developmental gaps between siblings

There were 2117 sibling dyads included in the final analysis, with reduced scores reported among second-born children vs firstborn children in multiple ASQ-3 domains at both 6 and 12 months. Differences at 6 months included the following:

• 1.9 points for communication
• 6.4 points for gross motor
• 5.9 points for fine motor
• 6.7 points for problem-solving
• 13.8 points for personal-social

Differences for fine motor and personal-social domains remained significant at 12 months, but the differences in other domains were no longer statistically significant at this time. Nearly identical findings were reported from sensitivity analyses.

Investigators also highlighted reduced parental engagement scores for second-born children, with fixed-effects models that adjusted for sex and birth indicators measuring a difference of -0.77 points. Overall, the results highlight that differences in cognitive and developmental outcomes between firstborns and second-borns appear in the first year of life.

“Parallel reductions in parental engagement for second-born infants are consistent with the resource dilution model, suggesting that reduced individualized parental time may contribute to early developmental variation,” wrote investigators.

The role of early interventions

Interventions are available to improve cognitive performance in children at a young age, as highlighted by Jeanette Stingone, PhD, assistant professor at Columbia University Mailman School of Public Health, in an interview with Contemporary Pediatrics.² According to Stingone, early intervention (EI) services can achieve maximum benefit by capitalizing on the brain’s developmental plasticity.

By strengthening foundational skills, EI services reduce the risk of small delays becoming larger gaps as children age. These services are also individualized, allowing children to receive precisely targeted support based on comprehensive evaluations.

“If you can get [delays] earlier when they’re still small and correct them, then a student or child can go on and learn with their peers,” said Stingone.

References

1. Tsuchida A, Matsumura K, Kasamatsu H, et al. Birth order differences in first-year neurodevelopment. JAMA Netw Open. 2026;9(3):e261265. doi:10.1001/jamanetworkopen.2026.1265
2. Stingone J. Jeanette Stingone, PhD, highlights the importance of early intervention for academic outcomes. Contemporary Pediatrics. February 17, 2026. Accessed March 17, 2026. https://www.contemporarypediatrics.com/view/jeanette-stingone-phd-highlights-the-importance-of-early-intervention-for-academic-outcomes