Sibling rivalry is usually intense but thanks to in-depth research it might come down to one simple reason: your sibling is a criminal at heart.
According to evidence from a Denmark and Florida study about birth order and delinquency from 2017, the order in which you're born matters when it comes to delinquencies in school, as a teenager and adult crime. They found that second-born boys are somewhere between 20-40% more likely to encounter disciplinary issues in school and run up against the criminal justice system than first-born boys.
Second-born siblings are only likely to develop delinquency-type behaviour when certain factors occur, with boys more likely than girls to develop such behaviour. If a child receives less attention from their parents than their first-born, or are not as treasured, they are likely to develop anger issues.
Authors of the study speculate that a second-born child may misbehave by copying the behaviour of their older sibling. The first-born child learns to act as an adult faster than the second-born child. Parents interviewed for the study admitted to being less passionate about raising their second or third child. That being said, first-borns have more alone time with their parents before their siblings come along.
The researchers have claimed that second-born children are just as healthy as first-borns:
We find no evidence that second-born children are less healthy, and indeed second-born children appear to be healthier at birth and have lower rates of disability in childhood. We also find no evidence that parents invest less in second-born children’s education. These children attend no-worse schools and are more likely to attend pre-kindergarten. We consider differences in parental attention as a potential contributing factor to the gaps in delinquency across the birth order. 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. 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.