A study of 240,000 Norwegian teenagers has found that first-born children are, on average, more than two I.Q. points more intelligent than the next kid in line.

As well, study author Dr. Petter Kristensen said in an interview that each successive child is likely to be a little less smart than the one before. A portion of Kristensen’s study, the largest ever of its kind, appears today in the journal Science.

The research drew its data from male Norwegian army conscripts aged 18 and 19. In Norway, almost all male teenagers are required to perform military service and IQ tests are also mandatory for most of these.

Kristensen says earlier research showed the same intelligence differences in female siblings. The Norwegian data does not shed any light on why the differences exist, although there are several theories, he says.

“The most dominant hypothesis has to do with the total parental resources that are not shared with anyone but the eldest,” says Kristensen, a professor at Norway’s National Institute of Occupational Health.

“When number two comes, there will be sharing of resources and less for each.”

Those parental “resources” are not likely related to the material goods or money allotted to the eldest child, but the amount of time parents can spend on such things as reading and playing. Kristensen says.

But greater access to parental attention cannot be the sole explanation for the elder child phenomenon as only children are less intelligent on average than the firstborns in larger families, he says.

“In fact ..... it’s quite clear that only children score lower than first children in two- or three-child families,” he says. “I’m not sure what the reasons for this are, but there have been (theories that) the older child gains from tutoring the younger children.”

Birth order expert Frank Sulloway, who wrote an analysis of the Norwegian work, says the 2.3- point IQ difference between first and second born children may appear modest, but could have significant life consequences.

He said, for example, that such a point spread could easily make the difference between getting into a first tier university or having to settle for second best.

Sulloway, a visiting scholar with the Institute of Personality and Social Research at the University of California, Berkeley, believes the IQ advantage enjoyed by older siblings finds its roots in the niche they often inhabit within the family. In particular, he says, their frequent role as baby sitter and teacher to their younger siblings builds a mindset, both internally and among their parents, that they are more responsible and intelligent than their brothers and sisters. This flattering mindset, in turn, translates into the elevated IQ score, Sulloway says.

Sulloway called the Norwegian paper the “most important” done in the field in the past 70 years and estimated its sample size was at least 50 times bigger than the next largest sibling order study. He also said it left the theory that the IQ advantage doesn’t exist “dead as a doornail”.

But Sulloway pointed out in an interview that the “elegantly designed” Norwegian research averages IQ levels out over an entire population and that younger siblings can and often do score higher than older ones in many families.

The Science paper suggests the differences in intelligence and birth order are not biological, as has been argued previously.

Some researchers, for example, have ascribed the birth order phenomenon to a growing maternal immune response to each successive child as it grows in the womb.

This heightened immune response, it was thought, may have attacked the child’s growing brain during pregnancy.

But the Norwegian study showed children who lost an older sibling and were raised as the “first born” enjoyed the same I.Q. advantage as actual first born children.

“It’s not plausible that it’s some biological or gestational factor, so what we’re left with is some kind of family interaction factors,” Kristensen says.