First borns really are smarter - but only by a single IQ point

First born children are smarter than their siblings - but only by a tiny margin, researchers have found.

The study of over 377,000 high school students concluded firstborns do have higher IQs and consistently different personality traits than those born later in the family.

However, researchers say, the differences between first-borns and 'later-borns' are so small that they have no practical relevance to people's lives.

'This is a conspicuously large sample size,' said University of Illinois psychology professor Brent Roberts, who led the analysis with postdoctoral researcherRodica Damian (now a professor of psychology at the University of Houston).

'It's the biggest in history looking at birth order and personality.'

The analysis found - as a previous large- scale study did - that first-borns enjoy a one-IQ-point advantage over later-borns, Damian said.

The difference is statistically significant but meaningless, she said.

The analysis is reported in the Journal of Research in Personality,

and also revealed consistent differences in personality traits between first-borns and laterborns - first-borns tended to be more extroverted, agreeable and conscientious, and had less anxiety than later-borns, for example - but those differences were 'infini- tesimally small,' amounting to a correlation of 0.02, Roberts said.

'In some cases, if a drug saves 10 out of 10,000 lives, for example, small effects can be profound,' Roberts said. 'But in terms of personality traits and how you rate them, a 0.02 correlation doesn't get you anything of note.

'You are not going to be able to see it with the naked eye. You're not going to be able to sit two people down next to each other and see the differences between them. It's not noticeable by anybody.'

The study controlled for potentially confounding factors - such as a family's economic status, the number of children and the rela- tive age of the siblings at the time of the analysis - that might skew the results, Damian said.

For example, wealthier families tend to have fewer children than other families, and so have a higher proportion of first-borns who also have access to more resources that may influence their IQ or personality, she said.

The findings confirmed those seen in the larger study, with specific differences between the oldest and a second child, and between second and third children. But the magnitude of the differences was, again, 'minuscule,' Roberts said.