“You can determine a person's personality by their birth order.”

The enduring notion that a person's personality is shaped by their birth order, dictating whether they are a responsible firstborn, a rebellious middle child, or a carefree youngest, largely stems from the work of Austrian psychotherapist Alfred Adler. In the early 20th century, Adler proposed that a child's position within the family profoundly influences their development, leading to distinct personality traits and behaviors. His theory captured public imagination, offering seemingly intuitive explanations for family dynamics and individual differences, and has since been a popular concept in everyday conversations and pop psychology.

Despite its widespread appeal, rigorous scientific investigation has largely debunked the idea that birth order significantly determines personality. Large-scale studies, analyzing data from hundreds of thousands of individuals across different countries, have found minimal to no substantive relation between birth order and broad personality traits such as extraversion, emotional stability, agreeableness, conscientiousness, or imagination. While some research notes a very slight, statistically significant advantage in intelligence for firstborns, this difference is typically only about one IQ point and is considered practically meaningless in daily life.

People commonly continue to believe in birth order's influence due to several factors, including confirmation bias and confusing age-related differences with birth order effects. It is easy to observe an older child acting more responsibly than a younger sibling and attribute this to their birth position, rather than simply their greater age and maturity. Additionally, parental approaches often differ for a first child compared to subsequent children, with first-time parents sometimes being more anxious or strict, which can create varying experiences within the family that are then mistakenly linked to birth order itself.