Study Says Eldest Children Have Higher I.Q.s

June 21, 2007
Study Says Eldest Children Have Higher I.Q.s
By BENEDICT CAREY
New York Times

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

The difference in I.Q. between siblings was a result of family dynamics, not biological factors like changes in gestation caused by repeated pregnancies, the study found.

Researchers have long had evidence that first-borns tend to be more dutiful and cautious than their siblings, early in life and later, but previous studies focusing on I.Q. differences were not conclusive. In particular, analyses that were large enough to detect small differences in scores could not control for the vast differences in the way that children in separate families were raised.

The new findings, which is to appear in the journal Science on Friday, are based on detailed records from 241,310 Norwegians, including some 64,000 pairs of brothers, allowing the researchers to carefully compare scores within families, as well as between families. The study found that eldest children scored about three points higher on I.Q. tests than their closest sibling. The difference was an average, meaning that it showed up in most families, but not all of them.

Three points on an I.Q. test, experts said, amount to a slight edge that could be meaningful for someone teetering between an A and a B, for instance, or even possibly between admission to an elite liberal-arts college and the state university, some experts said. They said the results are likely to prompt more intensive study into the family dynamics behind such differences.

"I consider this study the most important publication 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 in Berkeley.

Dr. Sulloway, who wrote an editorial accompanying the study, added, "There was some room for doubt about this effect before, but that room has now been eliminated."

Joseph Lee Rodgers, a psychologist at the University of Oklahoma and a longtime skeptic of the birth-order effect, disagreed, arguing that the new analysis was not conclusive. "Past research included hundreds of reported birth order effects" that were not legitimate, he 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 known to affect scores, including parents' education level, birth weight and family size, the researchers found that eldest children scored an average of 103.2, about 3 percent higher than second children and 4 percent higher than the third-born children.

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 gender 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 study. His co-author was Dr. Tor Bjerkedal, an epidemiologist at the Norwegian Armed Forces Medical Services.

To test whether the difference could be caused by biological factors, the researchers examined the scores of young men who had become the eldest in the household after an older sibling had died. Their scores came out the same, on average, as those of biological first-borns.

"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 I.Q. scores. First-borns 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, dumb down 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 older child's head start with mom and dad.

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, could benefit the teacher more than the student.

Another potential explanation concerns how individual siblings find a niche in the family. Some studies find that both the older and younger siblings tend to describe the first-born as more disciplined, responsible, a better student. 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 eldest siblings. 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 in general than first-borns, 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 astronomer who determined that the Sun, not the Earth, was the center of the planetary system, grew up the youngest of four. René Descartes, the youngest of three, was a key figure in the scientific revolution of the 16th century.

First-borns have won more Nobel Prizes in science than younger siblings, but often by advancing current understanding, rather than overturning it, Dr. Sulloway argued. "It's the difference between every-year or every-decade creativity and every-century creativity," he said, "between creativity and radical innovation."