WASHINGTON — Boys at the top of the pecking order — either by birth or because their older siblings died — score higher on IQ tests than their younger brothers.

Norwegian researchers report that it isn’t 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.

Mr. Kristensen of Norway’s National Institute of Occupational Health and Mr. Bjerkedal of the Norwegian Armed Forces Medical Services 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. 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.

“These two researchers demonstrate that how study participants were raised, not how they were born, is what actually influences their IQs,” said Frank Sulloway of the Institute for Personality and Social Research at the University of California, Berkeley.

The elder child pulls ahead, he said, perhaps as a result of learning gained through the process of tutoring younger brothers and sisters.