Does being the oldest or youngest sibling really shape your personality?

Are you a responsible oldest child, an overlooked middle, or a free-wheeling baby? For those who adhere to the theory that birth order influences personality, the answer to that question may hold the key to who you are as a person. At parties, family dinners, and therapy sessions, people can use birth order as a kind of shorthand for personality traits—an only child’s selfishness, perhaps, or a middle child’s struggle for visibility.

But though your personal experiences may very well indicate that birth order forms the personalities around you, psychologists beg to differ. Here’s why it might be time to drop the stereotypes.

Origins of the psychological theory of birth order

The idea that birth order influences a child’s personality might be as old as people themselves. After all, various societies have long privileged—or overlooked—people based on where they stand in their family.

In many ancient societies, for example, the arrival of a first child—and thus a parent’s transition to the head of a family—often translated to a higher social status. It also gave rise to ceremonies like special baths for first-time mothers in Micronesia and the traditional pidyon haben ceremony in Judaism, during which a first-born son is “redeemed” by paying five silver coins to a priest.

(Siblings can have surprisingly different DNA ancestry. Here’s why.)

Birth order has also long determined inheritance rights and royal lines of succession, as in the British monarchy which has long demanded a first-born “heir” and one or more “spares” as backups should something happen to the heir.

But the psychological theory of birth order didn’t develop until the early 20th century, when psychologist Alfred Adler theorized that birth order influenced not just social status, but a child’s development and personality. Known as the father of individual psychology, Adler theorized that an individual’s “family constellation” results in predictable personality traits. “The position in the family leaves an indelible stamp upon the individual’s life style,” Adler wrote in 1931.

According to Adler, the birth of a sibling deprives oldest children of their parents’ undivided attention—and as a result they are neurotic, more prone to conservatism, and inclined to imitate their elders. Second children are competitive attention-grabbers, while youngest children are pampered and lazy. Finally, he theorized that people who grow up without siblings have a “mother complex” and are in rivalry with their father.

Famous for his international lectures, popular psychology texts, and psychotherapeutic techniques, Adler’s influence still resounds throughout the field of psychology—and as a result, generations of psychologists undertook research that attempted to prove his theory of birth order.

What the research actually says about birth order

Studies conducted since Adler’s time have found associations between birth order and everything from educational attainment to sexuality to middle children’s success in team sports.

Frank Sulloway, one of the theory’s most prominent modern advocates, looked at adults and their careers in the 1990s and 2000s to assess the influence of birth order. He found a tendency for conservative research among famous firstborn scientists, with more radical research, such as the theory of evolution and relativity, more common among famous scientists born later in their family order. He also found differences between military and political strategies among militant firstborns like Maximilien Robespierre and moderate, nonviolent methods among famous middle-borns.

But the studies that most applicable to personality development look at the “big five” personality traits of openness, conscientiousness, extroversion, agreeableness, and neuroticism. And more recent birth order studies throw cold water on the theory that your birth order can shape your personality.

(Not an extrovert or an introvert? There’s a word for that.)

Rodica Damian, an associate professor of psychology at the University of Houston, conducted one of the largest such studies in 2015, using data from a longitudinal study of over 440,000 U.S. high school students. After controlling for socioeconomic status, sex, and age, the study showed that “the association between birth order and personality traits is as close to zero as you can get,” she says.

Another 2015 study underscored Damian’s findings: After analyzing three nationally representative samples from the U.S., Great Britain, and Germany, researchers wrote “we consistently found no birth-order effects on extraversion, emotional stability, agreeableness, conscientiousness, or imagination.”

But both teams of researchers found evidence for one trait that would please firstborn children (and dismay their younger siblings): The studies each showed that firstborns were slightly more likely to have high verbal intelligence.

This doesn’t necessarily mean that firstborns are smarter or learn more easily, Damian says. More likely it’s because firstborns spend more time around adults in their early childhood—and she points out that in her study, the difference was a matter of a single IQ point.

Overall, the other study team wrote, “we must conclude that birth order does not have a lasting effect on broad personality traits outside of the intellectual domain.”

What really makes a personality?

As a scientist, Damian is cautious about claiming that any theory has been “disproven.” However, she says that modern research essentially debunks the theory that birth order affects personality—which she calls a “zombie theory” because it just won’t die. So why does the idea still enchant the public—and why do researchers continue to plumb the question?


“Everybody has an opinion on it because everybody has a birth order, even only children,” says Damian. And part of the reason we just can’t quit birth order psychology may have to do with our own experiences that will always appear to support it. Older children will always seem to be more responsible and sophisticated than their younger siblings because they are more developmentally mature.

(How much does your name influence your future? It might surprise you.)

“Even though you see this and it’s true, you don’t have a magic lens to go back in time and observe the children at the [exact] same age,” Damian explains. It’s a “perfect confound,” she and her colleagues write—and it’s “one circumstance where personal experience will be wrong and the truth can only be discovered through good scientific reasoning and investigation.”

In truth, the science of personality development is anything but settled. Modern research using twin studies suggests that personality formation is about 40 percent due to genetics. The rest may be a matter of a complex combination of environment and cultural practices that help shape the disposition with which we’re born.

Though researchers can measure the “big five” personality traits, it’s harder to quantify the subjective experiences that shape our everyday lives and, perhaps, our personalities. Damian is currently studying the possible effects of people’s life narratives—the stories they tell themselves about their own experiences—on the people they become. But for much of the public, teasing out the complex web of nature and nurture is far less fun than teasing our siblings.

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