Research has shown that only children’s brain develops differently from other children who grow up with siblings. The absence of a brother or sisters leads to higher scores in creativity in later life, but lower scores in agreeableness. This is due to the fact that only children have to come up with things to do by themselves, whereas siblings can play with each other. By solving arguments with siblings, children develop agreeableness more easily. Interestingly, this difference can now actually be detected in college students’ brain structures. Hence, family constructions play an important role in the development of character and even brain structure.
source: http://www.telegraph.co.uk/science/2017/05/12/child-makes-creative-less-agreeable/