If you’ve ever wondered why the younger sibling always seems to push the boundaries just a little bit more — you might not be imagining it.

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A major study led by economist Joseph Doyle has found that second-born children, particularly boys, are more likely to get into trouble compared to their older siblings — and the reasons might come down to something surprisingly simple.

The research, which analysed large datasets from places like Denmark and Florida, found a consistent pattern across very different environments.

Second-born boys were between 20% and 40% more likely to be disciplined in school or come into contact with the criminal justice system than first-borns.

In some cases, the gap was even more striking. In Florida, second-born boys were up to 72% more likely to be suspended from school, while in Denmark, they were 40% more likely to receive a prison sentence by age 21 (when compared to their first-born brothers).

But despite those figures, the explanation isn’t as dramatic as it might sound.

Instead, researchers point towards subtle differences in upbringing and family dynamics.

MIT economist Joseph Doyle explained to NPR: “The firstborn has role models, who are adults.

“And the second, later-born children have role models who are slightly irrational two-year-olds, you know, their older siblings.”

It’s a simple but telling observation — younger children often grow up watching and mimicking someone closer to their own level, rather than adults.

At the same time, parenting styles tend to shift.

The study suggests that parents are typically stricter with their first child, while becoming more relaxed with the second. That combination — less rigid boundaries and a more unpredictable role model — could help explain the behavioural differences.

Doyle added: “Both the parental investments are different, and the sibling influences probably contribute to these differences we see in the labor market and what we find in delinquency.”

He also acknowledged how difficult it is to separate those factors completely.

“It’s just very difficult to separate those two things because they happen at the same time.”

Interestingly, the research ruled out several other potential causes. Factors like health at birth, school quality, and external environment didn’t explain the difference — especially since the comparisons were made between siblings from the same family.

Instead, one key factor stood out: time.

First-born children tend to receive more one-on-one attention from parents, particularly in early developmental years. When a second child arrives, that attention is naturally split — but not always evenly.

That early imbalance may affect important skills like emotional regulation and impulse control, which are closely linked to behaviour later in life.

Still, researchers are clear about one thing — this is about probability, not destiny.

The difference, while consistent, is relatively small, and experts stress that birth order doesn’t define who someone becomes.

Remember, every child develops their own personality, shaped by far more than just where they sit in the family lineup.

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