In 1996, American psychologist Frank Sulloway published a book called "Born to Rebel." In it, he described how children within a family fill "niches" that aren't yet filled by other family members – and therefore differ in personality and intelligence.
Oldest siblings are intelligent perfectionists, middle-children are more socially minded, and the baby of the family is more likely to rebel, he wrote.
To test this theory, researchers from Leipzig University and Mainz University analysed data from more than 20,000 respondents to national surveys in the USA, UK and in Germany, reports Süddeutsche Zeitung.
"We didn't find those effects described by Sulloway," said Julia Rohrer, lead author of the study.
In terms of extraversion, emotional stability, tolerance and conscientiousness, it doesn't seem to make any difference whether you're an older or younger sibling, the researchers claim.
There's only one thing that decreases slightly in younger siblings, they found – and that's openness to new experiences.
As well as personality, researchers also used IQ test results to compare intelligence levels between older and younger siblings.
Here, they did find a difference. With every subsequent sibling after the firstborn, average IQ scores dropped.
Younger siblings shouldn't worry too much, though – with scores between siblings just 1.5 IQ points apart, the difference is "miniscule," according to Rohrer.
The team aren't sure why exactly IQ levels differ between siblings. However, these results "don't mean that firstborn children are always cleverer," Rohrer said. "This is just an average."
In families with two children, the oldest child was the most intelligent in around 60 per cent of cases. In all other cases, it was the younger sibling who scored higher.