A study published in the October 2015 issue of the Journal of Research in Personality shows that the order in which a family’s children are born does not significantly affect the children’s personality or intelligence.

University of Illinois psychology professor Brent Roberts conducted the analysis along with University of Houston assistant professor of social psychology Rodica Damian, who was a postdoctoral researcher at the time.

“We were motivated to conduct the study because we are interested in the developmental experiences that lead to personality development,” Roberts said in an email. “Birth order is one of the most common reasons given by both researchers and laypeople about why people possess certain personality traits, so we thought it would be a good idea to test.”

Damian said they did not collect the sample on which the study is based. Instead, they received data collected by the American Institutes of Research with a sample size of 377,000 high school students.

She said this archival sample is called Project Talent and is publicly available for research purposes.

The psychology professors’ research paper, which can be found online, stated that high school students who were only children or were twins, triplets or quadruplets were excluded from the study because that is a common recommendation in birth order research.

“We realized early on that the data contained enough information on birth order to do a good study of its relation to personality and intelligence,” Roberts said.

In order to control for potential confounds, the study relied on a between-family design and many background factors, such as age, socioeconomic status and family structure.

According to the research paper, the study found there is a very small association between birth order and personality. The average absolute partial correlation was .02.

For birth order and intelligence, the average absolute partial correlation was .04, which is higher than the association between birth order and personality but is still very small.

“Social scientists often use statistical significance as a guide to determine whether an effect exists, but statistical significance is not only the result of an effect existing but is also affected greatly by sample size,” Roberts said. “Therefore, with the study’s huge sample size, even the tiny effects we found were statistically significant.

“We had to rely on the magnitude of the effects as a way to interpret the meaning of the results. It turned out the magnitude of the results was very, very small,” Roberts said.

Damian said it took about a week to analyze the data provided, but the paper was written about two years ago.

“The whole publication process takes a while,” she said. “You start to analyze the data, then you write the paper and then you submit it—then get rejected.”

Roberts said reviewers of the paper were not always happy with the professors’ findings and made it difficult to publish their work.

“The most challenging part of the study was responding to negative reviews from scientists who still believe that birth order has a noticeable relation to personality and intelligence,” he said.

He said birth order is a “perfect fulcrum for understanding why we should not rely on anecdotal experience over science” because it is almost everyone’s experience that there are birth order effects on personality, but that is mostly because birth order is “perfectly confounded with age.”

Roberts explained that because firstborns are always older, that gives them the chance to develop more than their younger siblings.

“That means that, on average, firstborns will be different than their younger brothers and sisters, not as a result of birth order but really because they are older,” Roberts said. “The only way to detect this fact is through examining large samples of kids and the appropriate statistical analyses using the best possible statistical controls.”

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Birth order has little impact on intelligence and personality, according to study.