A new study has found that eldest children tend to be more intelligent than their younger siblings.

The research from Leipzig University in Germany suggests that each new sibling is likely to be slightly less intelligent than the child that came before them.

It looked at the question of whether a person's position in the family has a lasting impact on their life course.

The study combined data from the United States, the United Kingdom and Germany - and found that firstborns score higher on "objectively measured intelligence".

It also saw a similar effect on self-reported intellect.

But it found no birth-order effects on extraversion, emotional stability, agreeableness, conscientiousness or imagination.

"This finding contradicts lay beliefs and prominent scientific theories alike and indicates that the development of personality is less determined by the role within the family of origin than previously thought", the study says.

"In our analyses, we confirmed the expected birth-order effect on intelligence", it adds.

But psychologist David Carey told Newstalk Breakfast the difference is relatively small.