The child raised as the eldest in a family is likely to have a higher IQ than their siblings, research shows.
Elder siblings may learn through teaching younger ones
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.
Professor Petter Kristensen, at the National Institute of Occupational Health in Oslo, and colleague Tor Bjerkedal, at the Norwegian Armed Forces Medical Service, said although the IQ difference they found in their study groups was small, it was significant.
The findings also suggested that the trend was down to social rather than biological differences, they said.
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.
Professor Kristensen said: "We found that it is the son's social position and not his biological position that counts."
Frank Sulloway, of the Institute of Personality and Social Research at the University of California, has been studying how upbringing influences personality and intelligence.
He told the Daily Telegraph the higher IQ in the first-born could, in part, be gained by their tutoring of younger siblings.
"In addition, the tendency for first-borns to occupy the niche of a surrogate parent, and to take on the role of the conscientious, self-disciplined and mature sibling may also explain why first-borns have higher IQs," he said.