The study, published in the journal Science, shows about a two-point gap in average IQ scores among firstborn men and men with living older siblings.
The study included nearly 244,000 teenage men in Norway. The men, who were 18-19 years old, took an intelligence test as part of Norway's compulsory military board examination.
The researchers included Petter Kristensen, MD, PhD, MSc, of Norway's National Institute of Occupational Health.
They noted whether the young men had any older siblings, including brothers or sisters who were stillborn or died in childhood.
Firstborn men had average IQ scores that were slightly higher than second-born men with living siblings. The same was true of second-born men and third-born men with living siblings.
But strict birth order wasn't the only important factor.
Birth Order Research
Men who had an elder sibling who had died had roughly the same IQ scores as firstborn children, the study shows.
Biological birth order (which includes all children in a family, including those who have died) and social birth order (which includes all living children in a family) may be equally important with respect to children's IQ scores, note Kristensen and colleagues.
The results held when the researchers considered other factors, including the parents' education, mother's age when she gave birth, and babies' birth weight.
However, an editorial published with the study points out that before age 12, younger children tend to outscore their older brothers and sisters on intelligence tests.
"This is because the younger sibling, being linguistically and cognitively less mature, degrades the firstborn's intellectual environment, whereas the older sibling enriches the secondborn's environment," writes editorialist Frank Sulloway, PhD, of the Institute of Personality and Social Research at the University of California, Berkeley.
That pattern seems to reverse over time, perhaps because older children get an intelligence boost from being their younger siblings' informal tutors in the ways of the world, Sulloway notes.
Calling the Norwegian study "elegantly designed," Sulloway says the greatest challenge is to find other large data sets to investigate other possible explanations for the findings.