Boys at the top of the pecking order(长幼次序) either by birth or because their older siblings(兄弟姐妹)died score higher on IQ tests than their younger brothers.
Norwegian researchers now report that it's a matter of what they call social rank in the family that gives the first born the highest scores or if the firstborn had died young, the next oldest.
Kristensen and Bjerkedal studied the IQ test results of 241,310 Norwegian men drafted into the armed forces between 1967 and 1976. All were aged 18 or 19 at the time.
The average IQ of first-born men was 103.2, they found. Second-born men averaged 101.2 but second-born men whose older brother died young scored 102.9. And for third-borns, the average was 100.But if both older brothers died young, the third-born score rose to 102.6.
The findings provide "evidence that the relation between birth order and IQ score is dependent on the social rank in the family and not birth order as such," they concluded.
It's an issue that has been hotly discussedsince at least 1874, when Sir Francis Galton reported that men in important positions were more likely to be firstborns than would have been statistically expected.
Since then, several studies have reported higher intelligence scores for firstborns, while other analyses have questioned those findings and the methods of those reports.
These two researchers prove that how study participants were raised, not how they were born, is what actually influences their IQs," said Sulloway, an American professor, who was not part of the research team.
"The elder child pulls ahead," he said, "perhaps as a result of learning gained through the process of teaching younger brothers and sisters."
"The older child benefits by having to organize and express its thoughts to tutor youngsters, "he said, "while the later born children may have no one to tutor."
