Does the order of birth really dictate things such as intelligence and personality? 

Perhaps not. 

According to a new study in the Proceedings of the National Academy of Sciences, oldest children in general have a slightly higher IQ than their younger siblings — but not by much. 

In addition, the research team led by Dr. Julia M. Rohrer of the University of Leipzig in Germany, found that birth order did not impact personality traits such as extraversion, agreeableness, emotional stability, conscientiousness, or imagination. 

The study involved a meta-analysis of surveys with data from more than 20,000 people. 

“I think it is important to keep in mind what the very small effects that we found actually mean,” Rohrer told Healthline. “They do not mean that every firstborn is slightly more intelligent than his or her younger siblings, but that, if you assess the intelligence of a large number of sibships, you will find slightly more sibships in which the firstborn is the smartest than sibships in which a latter born is the smartest.” 

There are much stronger associations between personality traits and, for example, gender, age, socioeconomic status, or culture, she said. 

But not so birth order. In fact, she said it’s difficult to predict what a person’s character will be, even with combining all factors that might affect personality. 

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Exploding the Niche Theory

The report refutes the popular Family Niche Theory, proposed in a 1996 study, which suggests links between birth order and many personality traits. 

Family Niche Theory posits that the firstborn child fills a more traditional niche, so they tend to be responsible, worry about parent-pleasing, and be more dominant. 

Additional children must fill a different niche — such as being rebellious, open to new experiences, or more sociable — to get attention from parents, according to that theory. 

Rohrer says some aspects of the niche theory make sense, such as children differentiating themselves from each other as they grow. For instance, perhaps an older sibling is good at math, so the younger sibling wants to take up singing to find an unoccupied niche. 

The specific ways in which they will differ, however, is not determined by their birth order, according to this latest research. 

A similar study with similar results to Rohrer’s was published earlier this year. 

Dr. Rodica Damian, a researcher on the University of Illinois study, explained the theories of birth order and personality compared to birth order and intellect. 

In short, the basic tenet in the Family Niche Theory is that older children are more intellectual because they’re surrounded by adults early on, so their environment is more intellectually rich. With each younger child added, the intellectual environment becomes diluted, she explained.   

“These ideas seem to be supported by the two new studies, as we did find small differences in IQ, especially in verbal intelligence, which suggest that yes, the parents may intellectually stimulate the firstborns more,” Damian told Healthline. 

She added the differences tend to be small but more significant than personality differences. 

“Even if differences in behavior may occur in the early family context, they do not translate into stable and permanent personality differences as it was originally predicted,” Damian said. 

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