Birth order barely touches your personality
The idea that firstborns are driven and middle children are diplomatic has felt like common sense for over a century. Three national datasets and 20,000 participants later, the picture is more complicated.
Frank Sulloway's 1996 book Born to Rebel made the case that birth order is one of the fundamental shapers of human personality, producing reliable patterns across five major traits: conscientiousness, agreeableness, openness, emotional stability, and extraversion. The theory was influential and intuitively satisfying.
In 2015, psychologists Julia Rohrer, Boris Egloff, and Stefan Schmukle published a study in PNAS using combined data from 20,000 participants in national surveys from the United States, Great Britain, and Germany. They found one confirmed birth-order effect: firstborns scored slightly higher on measures of intelligence and intellectual curiosity — roughly 1.5 IQ points per birth-order step, a real but small difference.
For the other four personality traits, the effect was consistently null. Not small — null. Firstborns were not measurably more conscientious. Later-borns were not measurably more agreeable or sociable. The patterns held across both within-family comparisons (siblings measured independently) and between-family comparisons, and across all three national samples.
Why do people believe it anyway? Probably because families create birth-order narratives — the responsible oldest, the wild youngest — and children grow up inhabiting those stories. The labels get reinforced until they feel like personality.
Birth order is real as a family mythology. As a determinant of who you actually become, the evidence is much thinner.