Oldest siblings show slight IQ advantage.

For more than a century, researchers have argued about whether first-born children tend to surpass their later-born siblings in intelligence. A large study now indicates that eldest sons indeed score slightly higher on IQ tests than boys with older siblings do.

This IQ effect reflects how participants were raised, not absolute birth order, say Petter Kristensen of the National Institute of Occupational Health and Tor Bjerkedal of the Norwegian Armed Forces Medical Services, both in Oslo. Later-born sons who became the eldest after an older sibling died in childhood displayed an IQ advantage comparable to that seen in firstborns.

Kristensen and Bjerkedal analyzed data from compulsory military examinations of 241,310 Norwegian men, ages 18 and 19, conducted from 1985 to 2004. The average IQ for all men, regardless of birth order, fell within the normal range. However, the average IQ for those who grew up as the eldest child--either as a first-born or because of older-sibling deaths--exceeded that of men with older siblings by 2.3 points, the researchers report in the June 22 Science.

Even that modest difference can substantially influence academic and professional achievement, argues Frank J. Sulloway of the University of California, Berkeley in an editorial published with the new study.--B.B.