One of the books concerning the psychology of children, known internationnally, is Tout ce joue avant 6 ans.
Well personnally, after babysitting lot of babies with different kind of families over France, I discovered that the facts in this book are right! Per example, for a child of monoparental family it is easier to live with it if the separation took place before the six years old while it has lot of impacts of children above this age!
I think that there is in us, a part innate! A part created by our parents that will always reflect in our personnality ( it is what was worked before our 6 years old), a part of our education after this 6 years old given by our parents, teachers, family, society and then our experience in life!
Our personnality is a real mix!
Regards,