Four in Ten

In 1996, psychologist Frank Sulloway published Born to Rebel, a historical analysis of nearly 6,000 figures and half a million biographical data points. His conclusion: firstborns defend the status quo, are conscientious and dominant, while laterborns reject authority, stay open to new ideas, and are more agreeable. In the book, the theory explained the Protestant Reformation, the French Revolution, and resistance to Darwin's theory. source: Born to Rebel: Birth Order, Family Dynamics, and Creative Lives

Twenty years later, Julia Rohrer, Boris Egloff and Stefan Schmukle tested the theory on over 20,000 people from three national panels, in Germany, the United States and Britain. The result: no trace of an effect on four of the five major personality traits, extraversion, emotional stability, agreeableness and conscientiousness. The only real effect, though small, showed up on intelligence. source: Examining the effects of birth order on personality

Rodica Damian and Brent Roberts confirmed the result on an even larger sample: 377,000 US high schoolers. The correlation between birth order and personality was 0.02. With intelligence, 0.04. Both are so small they explain under one percent of the real differences between people. source: The associations of birth order with personality and intelligence in a representative sample of U.S. high school students

The firstborn's average intelligence edge is real, but tiny. Stefan Schmukle summed up how tiny: in four of ten families, the younger sibling is still smarter than the older one. source: Examining the effects of birth order on personality

The intelligence gap, though small, is real and repeats across several large samples. The best-supported explanation is about resources: parents have more time, attention and means for a first child, before the family grows.

The studies cited here come from Germany, the US and Britain, wealthy countries with relatively small families. In a family with eight children, or in a subsistence economy, parental resources split differently, and the effect could look different.

A 0.02 correlation on personality means birth order explains under a hundredth of a percent of the real variation between people. The rest, almost the entire difference, comes from somewhere else: genes, friends, teachers, chance.