The New York Times (pg. A1)  [Printer-friendly version]
June 22, 2007

RESEARCH FINDS FIRSTBORNS GAIN THE HIGHER I.Q.

By Benedict Carey

The eldest children in families tend to develop higher I.Q.'s than
their siblings, researchers are reporting today, in a large study that
could settle more than a half-century of scientific debate about the
relationship between I.Q. and birth order.

The average difference in I.Q. was slight -- three points higher in
the eldest child than in the closest sibling -- but significant, the
researchers said. And they said the results made it clear that it was
due to family dynamics, not to biological factors like prenatal
environment.

Researchers have long had evidence that firstborns tended to be more
dutiful and cautious than their siblings, and some previous studies
found significant I.Q. differences. But critics said those reports
were not conclusive, because they did not take into account the vast
differences in upbringing among families.

Three points on an I.Q. test may not sound like much. But experts say
it can be a tipping point for some people -- the difference between a
high B average and a low A, for instance. That, in turn, can have a
cumulative effect that could mean the difference between admission to
an elite private liberal-arts college and a less exclusive public one.

Moreover, researchers said yesterday that the results -- being
published today in separate papers in two journals, Science and
Intelligence -- would lead to more intensive study into the family
dynamics behind such differences. Though the study was done in men,
the scientists said the results would almost certainly apply to women
as well.

"I consider these two papers the most important publications to come
out in this field in 70 years; it's a dream come true," said Frank J.
Sulloway, a psychologist at the Institute of Personality and Social
Research at the University of California, Berkeley.

Dr. Sulloway, who was not involved in the study but wrote an editorial
accompanying it, added that "there was some room for doubt about this
effect before, but that room has now been eliminated."

Effects of birth order are notoriously difficult to study, and some
critics are still dubious. Joseph Lee Rodgers, a psychologist at the
University of Oklahoma and a longtime skeptic of such effects, said
the new analysis was not conclusive.

"Past research included hundreds of reported birth order effects"
that were not legitimate, Dr. Rodgers wrote in an e-mail message.
"I'm not sure whether the patterns in the Science article are real or
not; more description of methodology is required."

In the study, Norwegian epidemiologists analyzed data on birth order,
health status and I.Q. scores of 241,310 18- and 19-year-old men born
from 1967 to 1976, using military records. After correcting for
factors that may affect scores, including parents' education level,
maternal age at birth and family size, the researchers found that
eldest children scored an average of 103.2, about 3 percent higher
than second children (100.3) and 4 percent higher than thirdborns
(99.0).

The difference was an average, meaning that it varied by family and
showed up in most families but not all.

The scientists then looked at I.Q. scores in 63,951 pairs of brothers,
and found the same results. Differences in household environments did
not explain elder siblings' higher scores.

Because sex has little effect on I.Q. scores, the results almost
certainly apply to females as well, said Dr. Petter Kristensen, an
epidemiologist at the University of Oslo and the lead author of the
Science study. His co-author was Dr. Tor Bjerkedal, an epidemiologist
at the Norwegian Armed Forces Medical Services.

To test whether the difference could be due to biological factors, the
researchers examined the scores of young men who became the eldest in
the household after an older sibling had died. Their scores came out
the same, on average, as those of biological firstborns.

"This is quite firm evidence that the biological explanation is not
true," Dr. Kristensen said in a telephone interview.

Social scientists have proposed several theories to explain how birth
order might affect intelligence scores. Firstborns have their parents'
undivided attention as infants, and even if that attention is later
divided evenly with a sibling or more, it means that over time they
will have more cumulative adult attention, in theory enriching their
vocabulary and reasoning abilities.

But this argument does not explain a consistent finding in children
under 12: among these youngsters, later-born siblings actually tend to
outscore the eldest on I.Q. tests. Researchers theorize that this
precociousness may reflect how new children alter the family's overall
intellectual resource pool.

Adding a young child may, in a sense, diminish the family's overall
intellectual environment, as far as an older sibling is concerned; yet
the younger sibling benefits from the maturity of both the parents and
the older brother or sister. This dynamic may quickly cancel and
reverse the head start the older child received from his parents.

Still, the question remains: How do the elders sneak back to the head
of the class?

One possibility, proposed by the psychologist Robert Zajonc, is that
older siblings consolidate and organize their knowledge in their
natural roles as tutors to junior. These lessons, in short, benefit
the teacher more than the student.

Another potential explanation concerns how siblings find a niche in
the family. Some studies find that both the older and younger siblings
tend to describe the firstborn as more disciplined, responsible, high-
achieving. Studies suggest -- and parents know from experience -- that
to distinguish themselves, younger siblings often develop other
skills, like social charm, a good curveball, mastery of the electric
bass, acting skills.

"Like Darwin's finches, they are eking out alternative ways of
deriving the maximum benefit out of the environment, and not directly
competing for the same resources as the eldest," Dr. Sulloway said.
"They are developing diverse interests and expertise that the I.Q.
tests do not measure."

This kind of experimentation might explain evidence that younger
siblings often live more adventurous lives than their older brother or
sister. They are more likely to participate in dangerous sports than
eldest children, and more likely to travel to exotic places, studies
find. They tend to be less conventional than firstborns, and some of
the most provocative and influential figures in science spent their
childhoods in the shadow of an older brother or sister (or two or
three or four).

Charles Darwin, author of the revolutionary "Origin of Species," was
the fifth of six children. Nicolaus Copernicus, the Polish-born
astronomer who determined that the sun, not the earth, was the center
of the planetary system, grew up the youngest of four. The
mathematician and philosopher Rene Descartes, the youngest of three,
was a key figure in the scientific revolution of the 16th century.

Firstborns have won more Nobel Prizes in science than younger
siblings, but often by advancing current understanding, rather than
overturning it.

"It's the difference between every-year or every-decade creativity
and every-century creativity," Dr. Sulloway said, "between
innovation and radical innovation."