Judith Rich Harris, author of The Nurture Assumption, explains Who We Are & Why, in her new book, No Two Alike:
Basically, Mrs. Harris believes there are three ‘perpetrators’ at work in the formation of the human personality, each associated with an aspect of a modular brain. One is the ‘relationship system,’ designed to maintain favorable relationships in society.Another is the ‘Socialization System,’ where the goal is to be a member of a group. The third is the ‘Status System,’ where we compete with our peers for status.
The interplay among these systems accounts for the emergence of differences between individuals. So it is that even identical twins develop different personalities because the members of their community see them as unique individuals and treat them differently. Their individual striving for status propels them into different modes of competing, which in turn differentiates their personalities.