Firstborns have higher IQs than their siblings
This short paper from Science reports tasty findings on the correlation between birth order and intelligence. While it’s generally accepted in the scientific community that older siblings generally have higher IQs due to different environmental influences, numerous possible factors make it tricky to prove causation from correlation. One criticism is that later siblings are likely to come from larger families, which relates to lower socioeconomic status and IQ.
A study of 241,310 Norwegians shows that sibling social order rather than biological factors is what causes the variation of intelligence in siblings. This study supports the popular confluence theory, which claims that intelligence is directly influenced by the intelligence level of the other family members. Thus, older siblings benefit from extra time spent with the parents, while younger siblings are negatively affected by the other children.
In nature vs nurture, nurture takes this round.
Kristensen, P. & Bjerkedal, T. (2007). Explaining the Relation Between Birth Order and Intelligence. Science, 316(5832), 1717. [PDF]