Why second-born sons are the naughtiest

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Second-born boys are more likely to get into trouble - both at school and with the law - than firstborn sons, finds a new large-scale study examining birth order and delinquency.

According to a recent report from Massachusetts Institute of Technology, second-born boys are 20 to 40 per cent more likely to be disciplined at school and to enter the criminal justice system compared to firstborns sons. And the reason why is simple - it's to do with the amount of attention they receive from their parents.

Researchers explored data from tens of thousands of sibling pairs from Denmark and Florida. The data incorporated measures of school performance and "delinquency outcomes" including truancy, suspension, criminal convictions and incarceration. Other factors, such as how long a mum took off for maternity leave, her employment status when her kids were aged 0 -10 years, and how old children were when they commenced school, were also examined.

"These societies have much different approaches to crime and punishment and are demographically very different," the researchers note of Denmark and Florida, "but our sets of findings are fundamentally quite similar."

Specifically, they add, "second-born boys are substantially more likely to exhibit delinquency problems compared to their older sibling." And in Denmark, second-born sons, "have substantially higher rates of juvenile crime, particularly severe violent crime, and imprisonment than do their older siblings."

And if you're a girl? "The gaps in delinquency are smaller when we investigate the effect of being a second-born girl," the authors write.

So the question is: why are these birth order effects occurring?

The research team were able to rule out a number of possible explanations for the difference, including factors related to health and education "We find no evidence that second-born children are less healthy," the authors note, adding that second-born kids were in actual fact, healthier at birth, with lower rates of disability in childhood than firstborns.

"We also find no evidence that parents invest less in second-born children's education," the team continues, explaining that second-borns were also more likely to attend pre-kindergarten.


Instead, the researchers point to the fact that second-born children tend to receive less parental attention than their older siblings - something those with multiple kids will understand all-too well. Additionally, firstborns reap the rewards of not only their own time with mum following their birth, but the time mum takes off with younger siblings, too.

"Second-born children tend to have less maternal attention than do their older siblings because first-born children experience their mother's maternity leaves and temporarily reduced labor market participation both following their own births as well as following the birth of the second-born," the authors explain.

"Therefore, in addition to the fact that first-borns experience undivided attention until the arrival of the second-born, we discovered that the arrival of the second-born child has the potential to extend the early-childhood parental investment in the first-born child."

It's not just about the parents, however.  Speaking to NRP, co-author Joseph Doyle said older siblings might just play a role too. Unfortunately, it's just a tad more difficult to study.

"The firstborn has role models, who are adults," Doyle said. " And the second, later-born children have role models who are slightly irrational 2-year-olds, you know, their older siblings. Both the parental investments are different, and the sibling influences probably contribute to these differences we see in labor market and what we find in delinquency. It's just very difficult to separate those two things because they happen at the same time."

The results, the authors argue, have important implications for social policy.

"Crime, delinquency, and incarceration have enormous social costs and are associated with major losses in human potential," they note.

"Our findings regarding systematically different dosages of early-childhood parental attention as a plausible mechanism [for the differences observed] also engender further discussion of parental leave as a long-run social benefit."