A Norwegian team found first born children and those who had lost elder siblings and had hence become the eldest, scored higher on intelligence. The link, reported in Science, was found by looking at more than 250,000 male Norwegian conscripts. Experts have disagreed for decades about how birth order might influence intellect and achievement.
Supporters of the theory argue the eldest child gets more undivided attention from their parents from an early age.Others claim differences occur in the womb before birth because with each subsequent pregnancy the mother produces higher levels of antibodies that may attack the foetal brain.While others claim the relationship between birth order and intelligence is false, being biased by family size - historically, couples with lower IQs have tended to have more children than couples with higher IQs.
For example, men who were third born but who then lost an elder sibling in early childhood and so were raised as the second born had IQ scores close to those of "genuine" second-borns.