There is no such thing as a typical firstborn, middle child or baby of the family according to a study that debunks the idea that personality is determined by birth order.

German researchers analysed data from 20,000 people from three nations in the most comprehensive and largest study to date on the issue.

They found that birth order had no effect on five key personality traits: extroversion, emotional stability, agreeableness, conscientiousness and imagination.

However, the paper, published in the journal Proceedings of the National Academy of Sciences, supported earlier findings that the first child in a family was likely to be more intelligent.

Co-author Julia Rohrer, from the University of Leipzig, said the link between birth order and personality was first mooted in the early 1900s by psychiatrist and philosopher Alfred Adler — the second of six children.

He claimed firstborns were privileged, but also burdened by feelings of excessive responsibility and a fear of dethronement and were more likely to score high on neuroticism.

However, the idea became firmly entrenched in the modern era when United States academic, Professor Frank Sullaway, developed the Family Niche Theory of birth-order effects in 1996.

Based on Darwin's theories of evolution, he argued that siblings adapted to certain roles within the family to reduce competition and enhanced the family unit's "fitness".

According to Professor Sulloway's theory, because firstborns were physically superior to their siblings at a young age, they were more likely to show dominant behaviour and become less agreeable.

Laterborns, searching for other ways to assert themselves, tended to rely on social support and become more sociable and thus more extroverted.

Personality theory deeply entrenched

Ms Rohrer said this theory had become deeply entrenched in the public psyche.

"Whether you have younger or older siblings appears to be of such great importance as a child, that the assumption that this has a lasting impact on personality just seems 'natural'," Ms Rohrer said.

"I think there are some biases at work that help firm those beliefs. For example, parents might infer their firstborn is emotionally unstable and very anxious because their infant cries a lot and is easily scared.

"The second-born child might actually cry just as much, but now the parents already know that this is just the way that children are, and stop attributing this behaviour to the child's character."

To test Professor Sullaway's theory, Ms Rohrer's team used data from three large national studies in Great Britain, the US and Germany.

The team undertook a range of analyses and looked for effects that were evident within families and also more generally expressed across all families.

"We tried our best, but we simply couldn't find the majority of the expected effects in our data sets," she said.

Their finding that birth order had no lasting impact on later personality traits was consistent across all three national studies, across the different measures of personality and across the participants' whole of life span, she said.

The study could be the final nail in the coffin of Professor Sullaway's theory.

Ms Rohrer said there was now a large body of work that had been unable to detect the birth-order effects as predicted by the Family Niche Theory.

"Rationally, we might want to abandon its main ideas or maybe modify its content in a way that it is more in line with empirical findings," she said.

Firstborn IQ effects 'rather humble'

Ms Rohrer said the study did confirm IQ is impacted by birth order and said it was likely this was due to social effects rather than biological.

"One theory is that later children 'dilute' the resources of the parents, including attention," she said.

While the firstborn gets full parental attention for at least some time, laterborns would have to "share" from the beginning.

Another possible contributing factor was that a firstborn could "tutor" their younger siblings, explaining to them how the world worked.

"Teaching other people has high cognitive demands," Ms Rohrer said.

"The children need to recall their own knowledge, structure it and think of a good way to explain it to youngsters, which could be a boost to intelligence for some firstborns."

However, she said the IQ effects were "rather humble" and not deterministic.

"The effect does not imply that every firstborn is slightly more intelligent than his or her younger siblings. It means that if you assess the intelligence of a large number of sibships, you will find more sibships in which the firstborn is smarter than sibships in which the laterborn is smarter," she said.

"So as a thirdborn, you could very well be more intelligent than your older siblings, and birth order is only one of multiple factors that can contribute to differences in intelligence amongst siblings."