Firstborns have higher IQ than younger siblings?

Firstborn children get more responsibility; treated as adults, leading to better IQ.

24 June, 2007: A study of 240,000 Norwegian men has found that eldest children have intelligence quotient (IQ) 2 to 3 points greater than younger siblings.

Firstborn sons have higher IQs than their younger brothers, and their social status within the family may explain why, say Norwegian researchers.

A research conducted by a group of experts has found that firstborn children are smarter than their siblings and that the reason for this is not genetics but the way their parents treat them.

The researchers added that heir findings should equally apply to women.

The study of 240,000 Norwegian men, published in the journal Science, found that the IQ of firstborns was 2 to 3 points higher than that of younger siblings. (The average IQ is 100.)

Though the difference may not sound much, experts are of the opinion that even a few IQ points could make a big difference over the course of a lifetime and set firstborns on a path to success.

Dr Petter Kristensen of Norway’s National Institute of Occupational Health – and a second-oldest son – says he did not believe in the ‘birth-order effect’ when he started his research, which was originally aimed at assessing the validity of IQ tests.

Dr Kristensen says his experience as a physician taught him that firstborns have lower birth weights and other health disadvantages. In medical studies, nearly all the differences favour younger children.

He undertook the research following a requirement of the Norwegian army that all conscripts undergo an IQ test. Kristensen looked at test results of all conscripts aged 18 to 19 between 1985 and 2004.

Frank J Sulloway, another researcher, who wrote a commentary accompanying the study, said 2 to 3 IQ points could translate to an added 20 to 30 points in a college entrance examination.

The research is the latest finding in a phenomenon that scientists have long noticed but have found hard to explain.

Year after year, more Nobel Prizes go to scientists and authors who are firstborn. Firstborns have been found to earn more than their share of National Merit scholarships and fill colleges in the United States in disproportionate numbers.

Eric Turkheimer, a researcher at the University of Virginia, the United States, say there are too many variables that shape an individual. His analysis found that firstborns had an average IQ of 103.2 – about 2 points higher than second-born males and about 3 points higher than men born third.

With these results in hand, Kristensen pursued a deeper question – what is the cause of this disparity?

Using the same data, he looked at second-born and third-born men who became the eldest in their families because of the death of one or two older siblings.

Kristensen found that those men had IQs close to that of firstborns, with second-born men at 102.9 and third-borns at 102.6.

The findings suggested that the mechanism behind the ‘birth-order effect’ is not biological but related to social interactions within families.

Kristensen concluded that older children are showered with attention early in life and treated as leaders in the family. They are handed more responsibility after younger siblings are born and live with higher expectations from their parents.

The results supported findings from an earlier study, published in February 2007 by the journal Intelligence. That study found the largest IQ gaps occurred in families that were relatively affluent or had well-educated mothers. The researchers were uncertain why these factors played a role.

Spacing between births also is a factor, according to Kristensen. Children born less than a year apart had the greatest IQ gaps. Differences in IQ scores lessened when there were more than five years between the first and second child.