Why eldest siblings are brainier

TL;DR

First-borns Outperform: First-born siblings achieve higher educational and income levels than younger ones on average.
Illness Gap Found: Younger siblings face two to three times higher risk of hospitalisation for severe respiratory illnesses in their first year, per Danish data.[[1]](https://www.economist.com/graphic-detail/2026/04/16/why-eldest-siblings-are-brainier)[[2]](https://www.hindustantimes.com/science/why-eldest-siblings-are-brainier-101776394856945.html)
Explains Wage Difference: Early sickness accounts for about half of the 1.9% wage gap between first- and second-borns; parental attention covers the rest.[[2]](https://www.hindustantimes.com/science/why-eldest-siblings-are-brainier-101776394856945.html)

The story at a glance

A graphic detail piece in The Economist reviews a new working paper using Danish administrative data on first- and second-born siblings born between 1981 and 2017. It argues that younger siblings suffer more from respiratory illnesses in infancy, brought home by older siblings, which hinders brain development and contributes to their long-term disadvantages in earnings and education. The article appeared on April 16th 2026, spotlighting this research amid ongoing interest in birth-order effects.[[1]](https://www.economist.com/graphic-detail/2026/04/16/why-eldest-siblings-are-brainier)[[3]](https://www.nber.org/system/files/working_papers/w29524/revisions/w29524.rev1.pdf)

Key points

First-borns outperform younger siblings in educational achievement and income, a pattern seen in prior studies like a 2005 Norwegian analysis of school dropouts.
No strong evidence links birth order to personality differences, such as eldest being responsible and youngest rebellious.
Younger siblings are two to three times more likely to be hospitalised for severe respiratory illnesses in their first year, especially in fall/winter or with close birth spacing.[[2]](https://www.hindustantimes.com/science/why-eldest-siblings-are-brainier-101776394856945.html)
Early illness harms brain development directly via inflammation or indirectly by diverting energy, leading to lower adult wages.
Sickness exposure explains roughly half of the 1.9% wage gap between first- and second-borns.
First-borns receive 20-30 more minutes of daily quality parental time throughout childhood, as parents divide attention evenly at any moment but new babies demand care.[[2]](https://www.hindustantimes.com/science/why-eldest-siblings-are-brainier-101776394856945.html)

Details and context

The research, by N. Meltem Daysal, Hui Ding, Maya Rossin-Slater and Hannes Schwandt, uses Danish data to show older siblings in childcare transmit viruses like respiratory syncytial virus (RSV) to vulnerable infants.[[3]](https://www.nber.org/system/files/working_papers/w29524/revisions/w29524.rev1.pdf)

Effects persist into adulthood, with causal links to reduced earnings, education, chronic respiratory issues and mental health problems for those exposed early.[[4]](https://www.nber.org/papers/w29524)

Parental investment dilutes over time: first-borns get undivided early attention, while later children share it, limiting stimulation during key brain-growth periods.

The article rules out personality as a driver, focusing instead on health shocks and family dynamics.

Key quotes

None sourced directly in visible article text or matching coverage.

Why it matters

Birth-order effects shape family outcomes and inequality, with first-borns gaining advantages in success metrics that compound over life. For parents, it highlights risks of close sibling spacing and virus exposure for infants, plus uneven attention allocation. Watch for peer review of the working paper and similar studies in other countries, though causal claims rely on quasi-experimental methods.[[3]](https://www.nber.org/system/files/working_papers/w29524/revisions/w29524.rev1.pdf)

FAQ

Q: Why are younger siblings more prone to respiratory hospitalisations?

A: Danish data show they are two to three times more likely than first-borns in their first year, as older siblings bring viruses home from childcare. The gap widens with fall/winter births or shorter spacing between siblings. This reflects intra-family transmission during infancy's vulnerable phase.[[2]](https://www.hindustantimes.com/science/why-eldest-siblings-are-brainier-101776394856945.html)

Q: How does early illness affect long-term outcomes?

A: It impairs brain development via inflammation or energy diversion, causing lower adult wages. The study finds a causal link, with exposed younger siblings facing reduced earnings and education. Prenatal fevers may add effects.[[2]](https://www.hindustantimes.com/science/why-eldest-siblings-are-brainier-101776394856945.html)

Q: What role does parental attention play?

A: First-borns get 20-30 more minutes of quality time daily over childhood, as parents split focus evenly but new infants demand care. This provides extra stimulation for early brain growth. It may explain the remaining half of the birth-order success gap.[[2]](https://www.hindustantimes.com/science/why-eldest-siblings-are-brainier-101776394856945.html)

Q: What is the wage impact of birth order?

A: Second-borns earn 1.9% less than first-borns on average. Illness accounts for half; parental behaviour the rest. This holds in the Danish data analysed.[[5]](https://x.com/TheEconomist/status/2045865045133840664)

[[1]](https://www.economist.com/graphic-detail/2026/04/16/why-eldest-siblings-are-brainier)