A recent study shows that second-born brothers are more likely to develop certain behavioral issues. 
The study, performed by researchers from several universities including MIT and University of Florida, followed thousands of brothers in Denmark and Florida. “Despite large differences in environments across the two areas, we find remarkably consistent results: In families with two or more children, second-born boys are on the order of 20 to 40 percent more likely to be disciplined in school and enter the criminal justice system compared to first-born boys even when we compare siblings,” the study’s authors wrote. “Across both of our locations, and across different estimation techniques, we find that second-born boys are substantially more likely to exhibit delinquency problems compared to their older sibling.” 
Second-born children in both countries also performed worse on reading tests. In addition, Danish second-born children performed worse on math tests, although this wasn’t the case for the Floridian children.
Scientists aren’t sure why second-born children exhibit more behavioral problems.
The study’s authors ruled out health as a factor, since second-born children appear healthier. Other non-factors included schooling decisions and maternal employment. 
One possible explanation is parental investment—both in terms of time and money. When parents have a single child, they can afford to lavish them with attention; this isn’t the case when additional children join the household, as the parents must split their time between their kids. With later-born children, parents are also less likely to provide early cognitive stimulation at home, according to the paper. In other words, parents are generally less likely to read to or play with their second-born children. The paper also makes the case that sibling influence may play a role. 
“Later children, unlike first-borns, will have older siblings as role models,” the authors wrote. “In addition, older siblings may also benefit from teaching younger siblings and acting as such a role model.”
But there is some good news for second-born kids.
Another study from the University of Cambridge found that later-born children benefit from the sibling influence. They develop stronger social skills as a result of the peer effect. “The traditional view is that having a brother or sister leads to a lot of competition for parents’ attention and love,” said Dr. Claire Hughes of the university’s Centre for Family Research. “In fact, the balance of our evidence suggests that children’s social understanding may be accelerated by their interaction with siblings in many cases.” 
“One of the key reasons for this seems to be that a sibling is a natural ally. They are often on the same wavelength, and they are likely to engage in the sort of pretend play that helps children to develop an awareness of mental states.” And if second children are less successful, they’re at least more likely to make friends. “Second siblings do better in our tests and children who have better social understanding go on to be more popular in later life,” said Dr. Hughes. There you have it: If you were born first, you’re more likely to be successful—but less likely to be popular.
