Researchers have tried to understand how siblings' birth order affects their competitiveness, intelligence, kindness and other personality traits for more than a century. Now, a new study [PDF] backs up what plenty of older siblings have long argued: their younger brothers are more prone to get in trouble. Here are five takeaways from the thought-provoking research.
1. SECOND-BORN SIBLINGS ARE MORE LIKELY TO GET IN SERIOUS TROUBLE.
The study focused mostly on older brother/young brother and older sister/younger brother sets of siblings. Among two brothers, the younger boys were found to be 20 to 40 percent more likely to be disciplined in school or get in trouble with the law compared to the older boys. As study co-author Joseph Doyle, an economist at MIT, told NPR, "I find the results to be remarkable that the second-born children, compared to their older siblings, are much more likely to end up in prison, much more likely to get suspended in school."
2. THE EFFECT WAS MORE PRONOUNCED FOR BOYS.
Doyle and his colleagues didn't find the same trend among second-born girls with older brothers or sisters. Boys and girls have different rates of delinquency; in this study, the average number of delinquent first-born girls in sister pairs was 54 to almost 100 percent lower than first-born boys in brother pairs. "The gaps in delinquency are smaller when we investigate the effect of being a second-born girl," they write.
3. THE RESULTS WERE SIMILAR IN DIFFERENT ENVIRONMENTS.
The researchers used birth registries in Denmark and in Florida that identified siblings so birth order could be determined. Then, they compared that data to school records, criminal databases, and medical or public health records. Despite differences in racial demographics, education levels, parental employment, and approaches to crime and punishment between the two locales, the researchers found that "second-born boys are substantially more likely to exhibit delinquency problems compared to older siblings" in both Denmark and Florida.
4. FAMILY DYNAMICS PROBABLY PLAY A PART.
Among the families studied, first-born and second-born siblings were equally healthy and achieved similar levels of education, so those factors did not play a large part in explaining the younger kids' propensity for trouble. Instead, the researchers suggest there is less maternal attention paid to second-born children. First-born children "experience their mothers' maternity leaves … both following their own births as well as following the births of the second-born." In other words, Jan Brady might have been right about her sister Marcia.
5. OTHER STUDIES HAVEN'T UNCOVERED THE SAME LINK.
Previous studies have found little connection between certain personality traits or intelligence and the order in which siblings were born. A 2013 paper suggested that "contrary to popular belief, the relationship between birth order and delinquency is spurious." When it comes to interpreting the effects of birth order, researchers are still—metaphorically, at least—fighting over the TV remote.