Our psychology textbook puts a lot on emphasis on the fact that shared environment does not factor into a person's personality. Twin studies and adoption studies have been performed and identical twins are likely to share many of their personality traits and adopted children adopt the personality traits of their biological parents, even though they may never have had contact with them. Genetics seems to be the key driving force of how a person acts, thinks, and behaves.
New evidence in the field of neuropsychology has recently emerged that supports the hypothesis that a person's traits are determined by his or her genes.
Researchers have discovered an oxytocin receptor gene (OXTR) that is linked to social traits. Oxytocin is a hormone known by some as the "love" or "cuddle" hormone because it has a role in birth and bonding between mother and child. There are two behaviour-linked alleles of the gene- G and A.
According to the research, people with both copies of the G allele tend to be more social, outgoing, and more interpersonally inclined than those who have only one G allele or two A alleles. People homozygous for G allele are also at lower risk of mental disorders such as autism or schizophrenia. I infer that the G variant of the biological receptor allows better binding of the hormone to the active site of the receptor, leading to the positive, prosocial attitudes displayed by people.
People with GG would probably score higher in the extraversion area of the Big Five, and score lower in Neuroticism. Researchers said that people could instantly detect the genotype by a first impression of the person based on how the person acts in front of another. I would classify myself as a GA, and my best friend as an AA, whereas many of my coworkers would be GGs.
Since this study is new, more research on the gene/protein product must be done in order to confirm the validity of their hypothesis. Little is known about how these differences manifest behaviorally and whether they actually are readily detectable by outside observers.