The child raised as the eldest in a family tends to have the highest IQ, according to a study published last night. The report appears to end years of academic disagreements over how birth order influences personality, intelligence and achievement.
Until now, the idea that sibling rank personality and brain power were dogged by contradictory findings but an analysis of Norwegian 18- and 19-year-olds settles the issue in favour of children who are raised as the eldest, says the journal Science.
Prof Petter Kristensen, at the National Institute of Occupational Health in Oslo, and Tor Bjerkedal, at the Norwegian Armed Forces Medical Service, found the link by studying the birth order, IQ and family situations of more than 250,000 Norwegian conscripts.
The effect does not link IQ and biological birth order because the team found that if a child has lost one or two older brothers or sisters, and they are reared as the eldest, their IQ is still higher by an average of two or three points than their two younger siblings.
Although this IQ difference is small, it is significant. For example, if there was a more prestigious college for students with IQs above the mean, and a less desirable institution for all other students, one commentator estimated that an eldest child would be about 13 per cent more likely than a second-born to be admitted to the better institution.
"We found that it is the son's social position and not his biological position that counts, and the only explanations plausible to me are factors related to parental attention and stimulation," Prof Kristensen said.
The effect is the same for boys as girls, he added.
"In our data set, it does not matter whether the young man being tested has sisters or brother, it is the number of elder siblings that count."
One reason for the higher IQ in first-born could be due to the tutoring that the elder children usually perform, according to Frank Sulloway, of the Institute of Personality and Social Research, University of California.
He has spent two decades studying how upbringing influences personality and intelligence.
"In addition, the tendency for first-borns to occupy the family 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 than their younger siblings," Prof Sulloway said.
