Birth order does not affect IQ

Macleans:

Researchers used to think older kids tended to be smarter than their younger siblings, but a new study suggests that's not the case. Instead, some faulty statistical work might be to blame for the earlier finding.

Most previous studies compared children from different families, so they were showing differences between large and small families, not differences between siblings, according to researchers led by Aaron Wichman of Ohio State University in Columbus.

They used data collected from 1986 through 1998 involving nearly 3,000 families who participated in the U.S. National Longitudinal Survey of Youth. All the children in the study took intelligence tests that measured skill in mathematics, reading recognition and reading comprehension. The data set allowed the researchers to compare children within a family, to see whether first-borns did better on these tests than their younger siblings.

The researchers also compared intelligence test results at two specific age points (seven to eight years old and 13 to 14 years old). Other studies had examined how children in a family scored on intelligence tests taken at one time, when children's ages may vary widely. This, they said, may also have affected study results.

"Birth order may appear to be associated with intelligence, but that's only because larger families don't have the advantages of smaller families," Wichman says in a statement. "It's not your birth order that is important: Family environment and genetic influences are the really important factors."

Intelligence not set by birth order, study says