The idea that sibling birth order affects personality has been around since the early 1900s, and became entrenched after Dr. Frank Sulloway published his book Born to Rebel in 1996.
A new study has debunked this theory. The study, published in the Proceedings of the National Academy of Sciences, has found “no birth-order effects on extraversion, emotional stability, agreeableness, conscientiousness, or imagination”. The study has, however, confirmed that first-born siblings have a higher IQ across a wide study sample, but the effect is slight and not universal.
This leads the study authors to conclude that:
On the basis of the high statistical power and the consistent results across samples and analytical designs, we must conclude that birth order does not have a lasting effect on broad personality traits outside of the intellectual domain.
Source: ABC Science.