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.