The question of how a brother or sister's intelligence ranks against other siblings has been fascinating the scientific community and been a source of competition within families "for over 100 years", but a group of researchers now say they have an answer.
A study by scientists from the University of Leipzig in Germany, has found the first-born child is often the most intelligent, with each successive child slightly less so.
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Dr Julia Rohrer and her colleagues found "the effect that firstborns score higher on objectively measures intelligence and additionally found a similar effect on self-reported intelligence".
Using data from over 20,000 adults from the US, Great Britain and Germany to find their conclusions, the research flies in the face of scientific theories which indicate the development of personality is determined minutely by the role within the family.
However, Rohrer does not believe the reason this result persists is because the intelligence is innate, but because it is developed through interactions within the family.
"A first-born can 'tutor' their younger siblings, explaining how the world works and so on," Rohrer said. "Teaching other people has high cognitive demands - the children need to recall their own knowledge, structure it and think of a good way to explain it - which could be a boost to intelligence for some firstborns."
A first-born will enjoy full parental resources, including attention, the second-born will have to share with the firstborn from the start, and it gets even worse for the third-born.
There were, however, no birth-order effects on "extraversion, emotional stability, agreeableness, conscientiousness, or imagination", only intelligence.