New research shows eldest children tend to be more intelligent than their younger brothers and sisters, because of the way they are raised.

Important influences may include the benefits of having a younger sibling to explain things to, or extra one-on-one attention from parents early in life.

The study, of almost 250,000 Norwegians, was able to resolve the nature-versus-nurture debate over intelligence and birth order because of its unusual design: it included young people whose older siblings had died as babies.

These people, who had been reared as an eldest child, performed like first-borns on intelligence tests, scoring on average more than two points higher on IQ tests than younger siblings.

Petter Kristensen, of the University of Oslo, said this would not have been the case if a biological factor, such as environmental influences in the womb, played an important role.

It showed IQ was "dependent on social rank in the family, and not birth order as such, " he said.

It has been known for more than 130 years that first-born men tend to be more successful but some studies showing first-borns were also more intelligent have been criticised as flawed. A University of California psychologist, Frank Sulloway, said the new study, of 18-year-old male conscripts, published today in the journal Science, was conclusive.

A University of Queensland psychologist, Professor Pat Noller, said eldest children also had "a lot more exposure to parents on their own in those early months", which could influence their intellectual development.

However, she said that in her research on twins' birth order the IQ difference was small and unlikely to alter important life events, such as the kind of occupation attained.

First-born brainier, or brighter later?

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