Why second-born sons might be the sibling most likely to stir up family drama – according to research

Led by economist Joseph Doyle of MIT, the research examined families in both Denmark and Florida, vastly different in culture and schooling systems, and the results were remarkably consistent: second-born sons were 20% to 40% more likely to be disciplined in school and come into contact with the criminal justice system compared to their older siblings.

The birth order effect is real, especially for boys

Importantly, these findings hold even when comparing siblings from the same family, meaning it's not down to external factors like school quality, parental income or healthcare. In other words, it's not about where a child grows up, but when they were born in the family.

The research tracked outcomes including suspensions from school, truancy, and even incarceration. In Denmark, second-born boys were 40% more likely to receive a prison sentence by age 21 than first-borns. In Florida, they were up to 72% more likely to be suspended from school.

So, what’s causing it?

The study examined a range of potential causes, from birth complications to differences in school quality, and ruled most of them out. Second-borns were actually healthier at birth on average, and attended schools of similar or even better quality than their older siblings.

What the researchers did find was a clear difference in parental time investment. First-born children tend to receive more one-on-one attention from their parents, especially between the ages of two and four. When a second child comes along, attention naturally gets divided, but interestingly, the firstborn often continues to receive more intensive parental care, even after the sibling’s arrival.

This unequal early investment may affect the development of non-cognitive skills, such as emotional regulation and impulse control, traits that are strongly linked to behavioural outcomes later in life.

The ‘middle child’ theory, backed by data

While psychology has long speculated on the challenges faced by middle children, this study gives hard numbers to back it up. Being the second-born, especially as a boy, appears to carry a higher risk of behavioural issues, not because of anything inherently “bad”, but because of the dynamics of family structure and attention.

The research also considered non-cognitive assessments in Denmark, where second-born boys scored significantly higher on hyperactivity scales by age 12. These behavioural indicators were present long before any formal school discipline, suggesting early developmental patterns are key.

Does this mean all second-borns are trouble?

Absolutely not. The researchers are clear: this is about risk, not inevitability. Not every second-born child will act out or struggle in school. But the patterns are strong enough that they should prompt a rethink of how parenting attention is distributed.

Understanding that second-born boys may need more direct engagement, not less, could help parents support their children more effectively, especially during those crucial early years.

What should parents take from this?

If you’re raising more than one child, it might be worth reflecting on how your attention and expectations are balanced between them. As the research shows, the timing and amount of parental involvement can have a big impact on behavioural development.

And while it’s easy to stereotype the “troublesome” middle child, this study encourages a more nuanced view, one that highlights how birth order can subtly shape childhood experiences in ways that matter.

So the next time your second-born kicks off at the soft play or refuses to come down from the tree they just climbed? Know that science sees you — and that with a little extra patience (and maybe a snack), things can level out.

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