Psychologist Alfred Adler believed it played a huge role in personal development.
The recent announcement that the Duke and Duchess of Cambridge are expecting a third child will mean big changes for the children.
Each will take on a new role – eldest, middle and youngest.
Does birth order make a difference to how you grow up?
Brendan Kelly, Professor of Psychiatry at Trinity College Dublin dispelled some of the myths with Ciara Kelly on High Noon.