You might have heard people say the oldest child in a family tends to do better in school or earns more later on, and it turns out there’s actually some truth to that.
Studies have found that firstborn children often score slightly higher on IQ tests, perform better in exams, and sometimes earn more money as adults. That doesn’t mean younger siblings aren’t smart or successful. The differences are usually small, and loads of younger siblings go on to do amazing things. What this research is really looking at is patterns across thousands of families, not judging individuals. Still, it raises a big question about why this pattern shows up so often.
Now that researchers at the University of Copenhagen have discovered a modest but still noteworthy 1.9% wage gap between siblings, it’s even more fascinating to discover why this might be.
Some of these differences show up much earlier than people think.
One of the most interesting parts of this research is how early the gap seems to begin. Scientists have found that differences between firstborns and younger siblings can already be seen within the first year of life. That’s long before school, homework, or even proper talking begins.
This is important because it suggests the cause isn’t just about effort or how hard someone studies later on. Instead, it points to early life experiences, health, and environment. When something shows up that early, researchers start looking at what babies are exposed to in their first months of life.
Germs and illness might play a bigger role than people expected.
The study behind this story suggests that younger siblings are exposed to more germs when they are babies. That’s mainly because older brothers or sisters bring bugs home from school, nursery, or playtime with other kids. This means younger children are more likely to get ill during important stages of brain development.
These illnesses are usually mild and very normal, but they can still have small effects. When a baby is fighting off infections, their body is using energy to recover instead of putting all of it into growth and development. In the long run, those small differences might add up in ways that slightly affect learning later on.
Firstborns often get more one-on-one attention early on.
Another reason experts often talk about is attention. When a firstborn child arrives, they get all of their parents’ time and focus. There are no other children competing for attention, so parents naturally spend more time talking, reading, and interacting with them.
Once more children come along, things change a bit. Parents are busier, there’s more noise in the house, and time has to be shared. It doesn’t mean younger children are ignored, but the level of focused, one-on-one interaction is often lower than what the first child experienced.
Parents also tend to be more cautious the first time around.
Many parents will admit they are more careful with their first child. They follow advice more closely, keep routines strict, and pay extra attention to things like diet, sleep, and learning activities. By the time the second or third child comes along, they’re usually more relaxed.
This isn’t a bad thing at all, but it can lead to slightly different early environments. Firstborns may experience a more structured start, while younger siblings grow up in a more flexible and busy setting. Those small differences can shape how children learn and develop.
It’s more about small averages than intelligence.
It’s really important to understand that this research is talking about averages across large groups, not individuals. A tiny difference in average scores doesn’t mean one child is “smarter” than another. It just shows a pattern that appears when scientists study lots of families together.
In real life, personality, effort, support, and opportunity matter far more than birth order. Plenty of younger siblings outperform their older brothers or sisters in school, careers, and life in general. These patterns are interesting, but they’re not a rule anyone is stuck with.
Growing up with siblings also brings its own advantages.
While firstborns might get a small early boost, younger siblings often gain different strengths. They tend to grow up around more social interaction, which can help with communication, teamwork, and adaptability. Being around older children can also push them to learn things more quickly in everyday situations.
In many cases, younger siblings develop strong problem-solving skills and independence because they’re used to figuring things out in a busier environment. These qualities don’t always show up in test scores, but they matter just as much in real life.
What this really tells us about childhood.
What this research really highlights is how important early life is. Small things like health, attention, and environment can shape development in ways people don’t always notice. It shows that even everyday factors, like catching more colds or sharing attention, can have a slight impact over time.
At the same time, it’s a reminder not to overthink it. Families are messy, different, and full of variation, and children grow in all kinds of ways. Birth order might nudge things slightly, but it never decides how someone turns out.
