It was once hypothesized by statistician and sociologist Francis Galton in the late 19th century, and later by psychiatrist Alfred Adler (a contemporary of Sigmund Freud's), that the order in which children are born could determine their occupations and personalities later in life.
This notion fell out of favor in the 1980s, but recently scientists have revisited the idea that the birth order of siblings could shape our personalities and behaviors in subtle ways.
Joseph Doyle, a professor of applied economics at the MIT Sloan School of Management, co-authored such a study in 2017. He told Newsweek, "We now have many large-scale, well-conducted studies from around the world showing that education, labor market, criminal justice, and other outcomes deteriorate with birth order."
Troublesome Second-Borns
After studying data on boys in Florida and Denmark, Doyle and his team concluded that second-born males in families with two or more children were between 20 to 40 percent more likely to be disciplined at school, compared with firstborns. The authors of the study, published in the Journal of Labor Economics, also said second children had a higher chance of being involved in the criminal justice system.
"My own study found very large increases in criminal justice involvement as birth order increased," said Doyle. "Again, that magnitude was surprising given that we compared children with their siblings, which controls for other characteristics of families."
Brainy Firstborns
A study published in 2007 found that firstborns have higher IQs than their younger siblings. The research, carried out in Norway and presented in the journal Science, involved a study of almost 250,000 military conscripts.
The team concluded that firstborns appeared to perform better on IQ scores. However, that outcome depended on an individual's social as well as biological rank in the family—for instance, children who grew up in families with deceased elder siblings, the authors said.
Commenting more broadly on whether evidence suggests birth order affects how a person turns out, co-author Petter Kristensen, professor emeritus at the National Institute of Occupational Health's Department of Occupational Medicine and Epidemiology, told Newsweek: "The effect on intellectual capacity has been statistically significant in a number of studies with sufficient design quality and statistical power. This population effect is small—mean difference approximately two IQ units between first and second-born."
He stressed that this effect seen in a population study is probably limited on an individual level. "This means that it would be rather dubious to extrapolate the population effect to the individual family, although this is done all the time." s
Family Ties
The effects appear to seep into family relationships too, as evidenced by a 2003 study published in Human Nature. The study had 245 undergraduates fill out questionnaires on their attitudes toward friends and family. Assessing the resulting data, researchers concluded that middle-borns prioritize their friends, while last and firstborns prioritize family. Middle children were also less likely to help family members in need.
Daredevil Siblings
Researchers at the University of California, Berkeley, showed in a 2010 study that younger siblings were more inclined to take risks. They analyzed existing studies on birth order, and looked at performance data on 700 brothers who played in Major League Baseball. Not only were younger brothers more likely to take risks, such as stealing a base, but they were better at batting overall. The team published their findings in the journal Personality and Social Psychology Review.
Not Set in Stone
It remains unclear what exactly underlies the associations, Doyle said. The most common explanation is that parents might be more invested in their first child and that firstborns may influence later-borns, suggested Doyle. However, he said it is very difficult for scientists to separately identify these factors accurately, as they happen at the same time.
And before the firstborns among us begin drafting an email to our younger siblings with the subject line "Scientific evidence proving I am better than you in every single way," it is important to consider how experts arrive at their conclusions. Such studies, like the ones outlined above, can't be taken as deterministic, experts agree.
Frank Sulloway of the University of California, Berkeley's Department of Psychology, the author of the study published in Personality and Social Psychology Review, told Newsweek it is "far from the case" that birth order creates the same outcome for each and every firstborn and each and every later-born.
In a 2015 study published in the journal PNAS, researchers at the University of Leipzig and the Johannes Gutenberg University of Mainz found that, at least when it comes to forming a personality, birth order has no effect on factors such as a person's extroversion, emotional stability, agreeableness, conscientiousness or imagination.
Study co-author Julia Rohrer, a doctoral candidate at the University of Leipzig, told Newsweek, "It is important to keep in mind that all important life outcomes—such as personality but also well-being, occupational success and so on—are determined by a multitude of different factors, including both genes, environmental factors and their interaction.
She went on: "But most of the effects that we psychologists are talking about are probabilistic in nature, and they can only account for small parts of variation in the population. It is pretty safe to say that one's birth order does not predetermine one's life course in any meaningful manner."
Doyle agreed, saying, "The studies show what happens on average: Plenty of later-born children do better than their older siblings. It is not written in stone at all."
