Contrary to popular belief, an individual’s personality is only slightly affected by their birth position among siblings, according to a study published in Proceedings of the National Academy of Sciences.

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New research suggests that birth order does not affect personality.

The findings contradict both lay beliefs and prominent scientific theories, and indicate that the development of personality is less determined by the role within the family of origin than previously thought.

The question of whether a person’s position among siblings has a lasting impact on personality has occupied scientists for more than 100 years. In the early 1900s, Alfred Adler stressed the need to understand individuals within their social context, which included, he said, birth order.

Adler was a middle child, and Freud, a firstborn, felt threatened by this theory. The dispute led to Adler’s resignation from the Psychoanalytic Society – and the opening of a new branch of psychology, the Society for Individual Psychology.

Such theories remain popular, but they are seen more nowadays in terms of siblings competing for maximum parental investment and developing strategies to increase parental attention by filling different roles within the family.

Thus, the firstborn supposedly fills the more “traditional” role, by being a responsible, dominating role model, who worries about parent-pleasing, making him or her more conscientious, extravert and neurotic; whereas the later-born fills the more “rebellious” niche, by being more original, easy going and sociable.

Beliefs held about birth position in the family include firstborns supposedly being perfectionists, while middle children develop a talent for diplomacy and those born last are expected to be rebellious.

Previous empirical research on the relation between birth order and intelligence has shown a slight decline in performance on psychometric intelligence tests from firstborn to last born, but research into the effect of birth-order on personality has remained inconclusive.

Psychologists from the universities of Mainz and Leipzig in Germany analyzed the data of more than 20,000 adults from Germany, the US and the UK.

They found that central personality traits such as extraversion, emotional stability, agreeableness and conscientiousness are not affected by birth-order position.

Small effects were found regarding intellect, with firstborns more likely to self-report a rich vocabulary and less difficulty understanding abstract ideas, confirming results of previous studies.

The team says this effect on intelligence replicates very well in large samples, but it is too small to be meaningful on the individual level. Moreover, despite the decline in mean scores on intelligence tests, 40% of later-born siblings are still smarter than their older brothers or sisters.

Prof. Stefan Schnukle, of Leipzig University, comments:

The real news of our study is that we found no substantial effects of birth order on any of the personality dimensions we examined. This does not only contradict prominent psychological theories, but also goes against the intuition of many people.”

Medical News Today published research showing that the IQ difference between first and last born children is insignificant.