Why so happens: a family one, parents one, and mental faculties at children the different?
There are the interesting researches proving, that intelligence of first-borns always above mental faculties of all subsequent children in a family. The impression as if on one married couple a certain limit of reason which is distributed between posterity is taken away is made.
Why so it turns out? One experts believe, that in a family parents give to younger children of much more attention. Them indulge, sponsor, as a result of their intellectual development it is a little was slow to solve independently in the absence of stimulus problems. At seniors other situation: frequently they are born at young pairs to which have no time to potter with the child especially. Therefore at big brothers and sisters high level of independence develops, ability to make the decision and to incur responsibility, and it very much promotes intellectual development. However, it only the theory.