Elder children - either by birth or because their older siblings died - score higher on IQ tests than their younger brothers or sisters.
The question of whether firstborn and only children are really smarter than those who come along later has been hotly debated for more than a century.
Norwegian researchers now report that it is not a matter of being born first, but growing up the senior child, that seems to result in the higher IQ scores.
Petter Kristensen and Tor Bjerkedal report their findings in Friday's issue of the journal Science.
It's a matter of what they call social rank in the family _ the highest scores were racked by the senior child - the first born or, if the first born had died in infancy, the next oldest.
Kristensen, of Norway's National Institute of Occupational Health, and Bjerkedal, of the Norwegian Armed Forces Medical Services, studied the IQ test results of 241,310 Norwegians 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 sibling died in infancy scored 102.9.
And for third-borns, the average was 100.0. But if both older siblings 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 is an issue that has perplexed experts since at least 1874, when Sir Francis Galton reported that men in prominent positions tend to be firstborns more often than would have been statistically expected.
Since then, several studies have confirmed higher intelligence scores for firstborns, while other analyses have questioned those findings and the methodology of those reports.
Frank J. Sulloway of the Institute for Personality and Social Research at the University of California, Berkeley, welcomed what he called the Norwegians' "elegantly designed" analysis.
"These two researchers demonstrate that how study participants were raised, not how they were born, is what actually influences their IQs," said Sulloway, 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 tutoring 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.