We already kind of knew this...

Science has confirmed it - the eldest child is the smartest. From the Trib:

Study finds firstborn kids smarter
Research supports nurture over nature
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By Robert Mitchum
Tribune staff reporter

June 22, 2007

Older brothers and sisters, there's a reason you're the smart one in the family.

A new study adds support to theories that nurture, not nature, is behind the long-observed tendency for firstborn children to be more intelligent than their siblings.

The study, published Friday in the journal Science, analyzed a database of a quarter-million men born between 1967 and 1986. Results indicate that environmental factors -- in this case a child's "social rank" among his siblings -- may determine intelligence levels.

Since the late 19th Century, scientists have wondered whether the order in which children are born affects their intelligence and personality. In 1973, a study of 400,000 Dutchmen found a relationship between birth order and intelligence, with average IQ scores decreasing from first-borns through younger siblings.

The new study, conducted by Petter Kristensen and Tor Bjerkedal, asked whether this relationship is due to biological or environmental effects. Using Norwegian government and military records, they compared first-, second- and third-born men with second- and third-born men whose older sibling or siblings died in infancy.

A difference of 2 IQ points

As predicted by previous studies, firstborn men performed better on a military intelligence exam than second-borns and third-borns by an average of 2 and 3.2 IQ points, respectively.

But second-born men with an elder sibling who died at a young age showed intelligence on a par with first-borns. Third-born men with one deceased elder sibling were similar in intelligence to second-borns, and those with two deceased older siblings had an average IQ more similar to first-borns.

The finding suggests that a child's "social rank" during upbringing, rather than birth rank, is predictive of IQ scores at ages 18 and 19. This observation rebuts theories that the decrease in intelligence with subsequent births is due to an unknown maternal effect of multiple births, which would have caused lower IQ scores in second- and third-borns regardless of an older sibling's death.

However, critics of the study questioned whether the authors' analysis obscured a multitude of other factors, such as parental intelligence and socioeconomic status.

"Birth order is notoriously difficult to study," Joe L. Rodgers, professor of psychology at the University of Oklahoma, said in a statement. "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."

New questions raised

The study opens the door to new questions about how social rank and birth order affects intelligence.

The confluence model, proposed by Robert Zajonc, argues that older children benefit from tutoring their younger siblings, resulting in higher intelligence in adulthood.

The dilution model, posited by sociologist Judith Blake, states that having more children stretches parental resources. Though firstborn children receive undivided attention until a younger sibling is born, all subsequent children must compete with their brothers and sisters.

Frank J. Sulloway of the Institute of Personality and Social Research in Berkeley, Calif., adds to this theory of limited resources by proposing that birth order may influence intelligence by creating Darwinian battles within the family.

"Lots of evidence suggests that first-borns and later-borns are pursuing different strategies for parental investment," he said. "Opportunities fall to elder siblings that involve responsibility and emulation of parental standards that are not open to younger siblings, so younger siblings look for other areas of expertise to shine in to augment parental investment."

So take heart, younger siblings. Though 2 IQ points may be a significant gap between a firstborn and second-born child, younger brothers and sisters find ways to compensate for a difference in raw intelligence.

"Two extra IQ points is bigger than most people realize, but it's not the end of the world," Sulloway said.

Copyright © 2007, Chicago Tribune

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