June 22, 2007 -- Given the choice, 9-year-old Carlyle Quinn would spend all day with his microscope, while younger brother Max would opt for the electric guitar.
There's no telling how these brothers, living in Brooklyn, N.Y., will grow up. But according to groundbreaking new research, it is very likely Carlyle has the highest IQ.
The study, published in the current issue of the journal Science, included more than 240,000 Norwegian men who took intelligence tests when they entered the military. In most families, the scores of the firstborn were three points higher than those of a second child, and four points higher than a third.
It's not a huge difference, but it might be enough to divide an A student from a B student.
"It is a breakthrough study that is very difficult to dispute methodologically," Frank Sulloway, a University of California psychologist who wrote an accompanying editorial to the study, told ABC News correspondent Bill Weir.
Lead study author Petter Kristensen, an epidemiologist at the University of Oslo, doesn't think the results apply only to Norwegians, but he added that it would be interesting to examine if family patterns are different in some way in Norway.
As for the reasons behind the apparent intelligence gap, Kristensen and other researchers insist it is not due to nature but nurture, with the firstborn enjoying the fruits of undivided attention early in life.
Intelligence Through Teaching
Experts believe that when older children tutor their siblings, it may help their brains grow.
Add to this the fact that in many families, if the eldest is organized and studious, the younger finds more creative ways to get attention.
Sulloway said such differences are not necessarily bad, and that such an arrangement might allow younger children to shine in different ways.
"Younger siblings may have three points of something else that doesn't show up on an IQ test," he said.
Exceptions to the Rule
Examine certain sets of famous siblings, and a tantalizing dynamic emerges that seems to support these findings. Studious Prince William and his playful brother Harry, for example. Or Bill and Roger Clinton.
There are exceptions, of course. Older brother George Bush was a prank-pulling C student, while Jeb hit the books.
And while firstborns like Einstein may have won more Nobel Prizes in science, some of the world's great innovators came late in the litter.
Thomas Edison? The youngest of seven.
Some researchers note that there are simply too many variables at work to pin something as complex as intelligence to just one or two factors.
"Birth order is notoriously difficult to study," said Joe Rodgers of the University of Oklahoma department of psychology in a statement issued this week in response to the study.
"The basic problem is that factors that differ between families (and there are hundreds of those) can cause the appearance of birth order effects when they are not really there," he said. "The Science piece is based on interesting and valuable data, but nevertheless leaves important methodological questions unanswered that raise the same kinds of doubts."
Moreover, previous studies have declared time and time again that there is no association between birth order and intelligence.
Still, the size of the Norway study is hard to ignore. And even skeptics hope the study will spark more research into how family dynamics shape our minds.
If nothing else, the study provides one more way to tease your little brother … or one more reason to resent your big sister.
ABC News correspondent Bill Weir contributed to this report.