In spite of sharing genes and environments, siblings are often not as similar in nature as one might think. But where do the supposed differences come from? Alfred Adler, a 19th- and early 20th-century Austrian psychotherapist and founder of individual psychology, suspected that birth order leads to differences in siblings.

Adler considered firstborns to be neurotic, because they don’t have to share their parents for years and are essentially dethroned once a sibling comes along. He also considered oldest children dutiful and sometimes conservative. According to Adler, the youngest children are ambitious, while middle children are optimally positioned in the family and are characterized by emotional stability. Adler himself was the second of seven children.

American psychologist Frank J. Sulloway, who, in the mid-1990s, combed history books for leading figures who were firstborns and rebellious ones who were born later, saw a similar trend. Among the later borns, he found lateral thinkers and revolutionaries, such as Charles Darwin, Karl Marx and Mahatma Gandhi. Among firstborns, he discovered leaders such as Joseph Stalin and Benito Mussolini. His explanation? Every child occupies a certain niche within the family and then uses his or her own strategies to master life. Firstborn and single children had less reason to quarrel with the status quo and identify more strongly with the worldview of their fathers and mothers. Younger siblings are less sure of their parents’ view and therefore more often choose alternative paths in life.

Such categorizations are popular because they’re rather intuitive, and one can always find an example of the sensible big sister or the rebellious young brother in their circle of acquaintances. As such, Adler’s words still appear regularly in educational guides and continue to reverberate in the minds of parents.

Furthermore, some studies confirmed the idea that sibling position can shape personality. For example, a 1968 study showed that, compared with later borns, first borns are less likely to participate in dangerous sports because of fears of physical injury. And a 1980 study of 170 female and 142 male undergraduates showed lower anxiety and higher ego in firstborns, as measured by the Howarth Personality Questionnaire. At times, however, these investigations used questionable methods. For example, members of the same family were often asked to assess themselves in terms of extraversion, openness to experiences, conscientiousness, tolerance and neuroticism. The catch is these surveys were conducted at only one point in time. The older siblings were therefore not only born first but also simply older. It has long been known that adolescents become more conscientious as they age. This trend could account for a large part of the results. Another methodological flaw was that only one person judged his or her own personality and that of his or her siblings. This detail is important because self-perception and the perception of others can sometimes differ considerably. In addition, the test subjects may have subconsciously incorporated the cliché of dutiful older siblings and cosmopolitan later borns into their evaluation and could have thus brought about the expected result themselves.

Meanwhile scientists who analyzed large, transnational data and compared different families with each other have found the effect of sibling succession on personality disappears almost completely. Researchers led by psychologist Julia Rohrer of the University of Leipzig in Germany evaluated data from more than 20,000 interviewees from Germany, the U.K. and the U.S. They compared the personality profiles of siblings but also of people with different birth orders who had never met. The Leipzig psychologists did not discover any systematic differences in personality.

In such studies, researchers must be particularly cautious because, in addition to age, the size of one’s family is another factor that’s intertwined with sibling position. A child from a family of four has a 50 percent chance of being a firstborn; the more siblings, the lower the probability. For example, the fact that many astronauts are firstborns does not necessarily speak to the special qualities of those born first. It’s likely that many astronauts come from smaller families. To better understand these influences, Rohrer and her team controlled forthe number of siblings. That’s because when there are more of them, there are more later borns. So the researchers hypothesized later borns may more often appear in families of lower socioeconomic classes—which could account for differences between children of different-sized families. 

The larger the sample, the more likely even very small effects will be detected. For example, in a 2015 study, which included 377,000 high school students, psychologist Rodica Damian and her colleague Brent W. Roberts, both then at the University of Illinois at Urbana-Champaign discovered that firstborns tended to be more conscientious, extraverted and willing to lead. Contrary to expectations, they were also more tolerant and emotionally stable than adolescents with older siblings. Yet the differences were very small, and the researchers concluded that the importance that is generally attached to sibling position in shaping one’s character is exaggerated.

“It is quite possible that the position in the sibling sequence shapes the personality—but not in every family in the same way,” says Frank Spinath, a psychologist at Saarland University in Germany. “In other words, there may be an influence but not a systematic one. Nevertheless, other influences weigh more heavily when it comes to the differences in character of siblings. In addition to genes, the so-called undivided environment also plays a role. For siblings who grow up in the same family, this includes the respective circle of friends, for example.” Further, parents do not treat their children the same regardless of their birth rank. Studies show that parents react sensitively to the innate temperament of their offspring and adapt their upbringing accordingly.

Damian’s study also found that on average, firstborns enjoy a small IQ advantage over their younger siblings. Those born first also tend to complete their education with a higher degree and opt for traditionally prestigious careers, such as medicine or engineering.

How does this intellectual advantage come about? Adler may be right that the undivided attention given to the first child in early life promotes cognitive abilities. This advantage is already apparent by the age of two. Norwegian researchers Petter Kristensen and Tor Bjerkedal cleverly showed that the difference in intelligence is not linked to biological factors (some had suspected it might be related to physical conditions during pregnancy). They tested children whose older siblings had died early. The researchers’ assumption was that although these children were biologically younger siblings, they assumed the role of the firstborn in the family. Compared with other younger siblings, they achieved better results in intelligence tests.