We've all heard it before: The oldest child is responsible and driven, the middle one is the peacekeeper, and the youngest is the fun-loving rebel. It sounds neat, maybe even a little true when you look at your family. But here's the twist: Science says it's mostly nonsense.

The idea that birth order shapes personality has been around for more than a century. It started with Alfred Adler in the early 1900s and stuck around because it feels right. Who hasn't met a bossy firstborn or a carefree youngest and thought, "Of course they act that way—they were born that way." But when psychologists tested the theory using real data, the results told a very different story.

As Judith Rich Harris (2002) put it, people cling to the idea because it makes emotional sense, not scientific sense. We like to believe family order explains behavior. That it gives our complex relationships a sense of order. Unfortunately, life and personality are much more complicated.

A major reality check came in 2015, when R. I. Damian and B. W. Roberts published a study using data from over 377,000 people. Their conclusion was blunt: Birth order barely matters (Damian & Roberts, 2015). Any differences between siblings, like firstborns seeming more conscientious, were so tiny they had no real effect on how people act in daily life.

That same year, another research team consisting of Johannes Rohrer, Boris Egloff, and Stefan Schmukle ran their own large-scale study, also finding no solid evidence that birth order predicts personality traits (Rohrer, Egloff, & Schmukle, 2015). The results........

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