The source of personality; most might think of this to be environment and family upbringing. But in fact genes play an equal role with upbringing to determine personality. And of the Big 5 personality traits, heritability's ranged from 42% to 57% with openness to experience being the most influenced by genes.
New Research on personality traits and twins suggests that heredity has more of an influence on personality than upbringing. The University of Minnesota has been studying twins since 1979, in that time over 350 pairs of twins have participated in a 6 day testing which includes analysis of blood, intelligence, brain waves etc. Their results showed the relationship between family and heredity. For example, a family of an innately timid child might tend to make that child more or less timid, but could not make that child brave. The study included 65 twins who were raised apart which determined the importance of heredity and of environment. When comparing identical and fraternal twins it showed that the identical twins were very much alike in personality despite being raised separately. This new study goes against both common sense and what has been agreed upon in the past by psychologists that environment is the key factor in personality.