All in all, the study found that second-borns presented more “challenging” behavior. While Second-born boys, in particular, were more likely to get in trouble at school and with the court system when they got older. They were even more likely to end up in prison.
The study finds: "We consider differences in parental attention as a potential contributing factor to the gaps in delinquency across the birth order,” the authors explained.
Doyle himself told NPR: “The firstborn has role models, who are adults. 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 the 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.”
(Disclaimer: the author's birth order resulted in no bias during the writing of this article. Sincerely, a firstborn child.)