However, they still found that people’s IQ and personality traits tended to differ slightly depending on birth order. They just found that in a larger study of over 377,000 high school students, that the difference is so minor that it doesn’t really mean anything.
Statistically, the results correlate with the classic stereotypes developed by psychologist Alfred Adler in the early 20th century: the firstborn as a natural leader; the spoiled, lazier youngest child; and of course the oft-ignored middle child. Adler’s theory has been debated among psychologists for years: some, like Frank Sulloway, maintain that Adler was largely correct. Others, such as Jules Angst and Cecile Ernst, claim the effects of birth order have been significantly exaggerated.
In real life applications, Prof. Roberts says, the differences in personality and intelligence are so minor that they’re ultimately insignificant. “You’re not going to be able to sit two people down next to each other and see the differences between them,” he continued. “It’s not noticeable by anybody.”