There is a theory followed by many psychologists, that the relationship between siblings, along with their self-esteem, might be genuinely affected by the birth order. In particular, defenders of the theory say, that older siblings usually have higher self-esteem and typically have a stronger personality than younger.
Falbo (1981) performed a study to prove the theory, that birth order might affect the level of a person’s self-esteem. He invited 1785 young people to participate in the experiment, where they had to complete several questionaries and personality instruments. He concluded, that firstborn children and older siblings usually have higher self-esteem and tend to compete for more, than younger ones.
There was another study, which aimed to prove that the birth order might affect the self-image of the person. Four scientists, Hubbard, Crockett, Gates, and Lineberger in 1988 started a study, where they were tracing the relation of birth order to self-concept of the person, along with a general tendency to depression or anxiety. The invited children aged 7 to 12, chosen from private and public schools. It is said, those children had all possible differences in reading level. At the end of the experiment, they concluded, that firstborn children have higher self-concept score, which was speaking for higher self-esteem.
There are several more studies performed to prove this theory. Each of them more or less confirms the idea, that the place one takes in the family has a significant impact on person’s self-esteem, self-image, ability to compete and a stronger personality.