We finally know why oldest children do so much better than their younger siblings

Opinion>Opinions - Healthcare The views expressed by contributors are their own and not the view of The Hill We finally know why oldest children do so much better than their younger siblings Comments: by Liberty Vittert Capito, opinion contributor - 06/19/26 7:00 AM ET Comments: Link copied by Liberty Vittert Capito, opinion contributor - 06/19/26 7:00 AM ET Comments: Link copied

My husband and I have three children. The oldest gets most of our time and attention — and that’s normal, according to a landmark new study released this week that tracked 1.2 million people over 40 years.

Oldest children also have a measurably better shot at higher incomes, better education and better mental health than youngest children. And for the first time, we have an idea of why.

Birth-order effects have been documented for the last 20 years. A major study in 2005 found that younger siblings fare worse than firstborns on lifetime earnings, educational attainment, mental health and, for women, teen pregnancies.

In fact, the further back in birth-order you fall, the worse the numbers get. This is a very scary and grim concept for parents, and no one had any idea why … until now.

The new study, which will be coming out in the American Economic Review, followed 1.2 million Danish children from birth to adulthood using government records — an incredible dataset. They found two very striking reasons for the birth-order gap, and they make sense intuitively.

Exposure to respiratory viruses in the first year of life, before the immune system is mature and before most childhood vaccines kick in, consistently predicted lower earnings, lower educational attainment and worse mental health decades later. The mechanism by which the younger sibling gets said respiratory illness is highly unfortunate: Older siblings bring home germs from daycare or school, and the younger siblings, who have no prior exposure or immunity, get walloped.

We know this because the data clearly show younger siblings are two to three times more likely to be hospitalized for acute respiratory conditions in their first year of life. After the first year of life (when younger children tend to start attending childcare), the hospitalization gap disappears. This makes clear that the culprit is the older sibling.

What does illness have to do with future income gaps? Roughly 85 percent of an infant’s calorie intake goes toward neural development. A serious infection will divert those calories to fighting illness instead. Moreover, disease exposure in the first six months of life produces income penalties three times greater than the equivalent exposure in the second half of the first year.

So disease exposure explains about half the lifetime outcome gap between older and younger siblings. The rest of the gap is explained by parental attention.

Firstborns receive 20 to 30 more minutes of quality time per day than the same aged younger siblings, amounting to roughly 3,000 hours over childhood. That’s a lot.

Now, are there any pitfalls to this study? The only one I think to be truly valid is generalizability. Denmark is not the U.S. Denmark has universal healthcare, parental leave and high vaccination rates. The ways in which early illness damages long-term outcomes could be mitigated by a family’s ability to follow-up with care.

But overall, there is more than enough evidence for us parents to take this study very seriously when it comes to having multiple children. Most of these studies about how your childhood determines your fate give you no actionable items and leave you feeling totally helpless. This one doesn’t.

This study says that time and disease are the two culprits for a very real problem. For disease, you can vaccinate — not just your infant but also yourself and your older children. You can also breastfeed where possible. Younger siblings who were breastfed for less than 6 months suffered significantly more acute infections.

To be clear, this is not a guilt trip for mothers who can’t breastfeed — I couldn’t breastfeed that long, but if you can, you should. And lastly, basic hygiene. Make sure the older kids are washing their hands, changing out of daycare clothes, and staying away from the baby if they are sick.

As for time, that one is completely within our control. Equalize your time. The 3,000 hour deficit is not fate — it is an average. Once you know about it, you can act on it and focus on not giving too much time to the older children compared to the younger.

For the first time, we have actionable insight into how to make all of our children’s lives better. That’s a pretty incredible move forward.

Dr. Liberty Vittert Capito is a professor of data science at Washington University in St. Louis and the resident on-air statistician for NewsNation, a sister company of The Hill.

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