Our earliest human relationships help to create a predictive model for all future human relationships. When the earliest relationships involve betrayal and trauma at the hands of other humans, it makes sense that a strong sense of distrust or even aversion will develop. Alternatively, if the earliest human relationships were positive, basic trust in others will be preserved or even enhanced.
This is classical conditioning, where a person's reaction to other people become a conditioned response. Positive early relationships lead to welcoming and seeking of new relationships. Negative early relationships lead to a generalized avoidance of and discomfort in relationship.
There are many questionable writings about innate “temperament” and “disposition” as though they were purely genetic. Human experience begins before birth. For example an infant in the womb can hear shouting and feel the effects of stress hormones. One can only discuss "innate" temperament by ignoring the research on prenatal experience.
[…] pups of stressed [rat] mothers […] were more fearful and irritable and produced more stress hormones. […] prenatally stressed monkeys […] result[ed] in a wide range of impairments including neuromotor difficulties, diminished cognitive abilities, and attention problems. […] Researchers hypothesize that a mother's stress hormones can damage the developing brain of the fetus. Very recent research shows that maternal stress hormones released during pregnancy may adversely affect human fetal brain development (Stien, Kendall, 2004, pp. 21-22).
Likewise shyness, often described as an innate trait, can better be described as a prior history of negative interpersonal experience. For example a scapegoat child in a family, targeted by fellow family members with criticism, rejection and sometimes violence, will come to expect that same treatment from society at large. That would make "shyness" a conditioned expectation.
Introversion, or the tendency to prefer solitude and avoid interaction with others, can likewise be traced to negative interpersonal experience. Just imagine that you're first exposure to human relationship is witnessing domestive violence. What would your expect from others? Of course, reality is more complex than this simple example. For example, just one loving figure can become a savior in a child's life.
In my clinical experience, I have found that those who experience betrayal and/or trauma in their first relationships (usually in their family of origin), are often introverted. They likely have a subconscious (or core) belief that their future relationships will be a repeat of prior negative experience. They will not have a conscious thought in their mind "I want to avoid other people." Instead, it will appear to be just an ordinary preference.
Beversdorf, D.1; Manning, S., et al, (2005). Timing of Prenatal Stressors and Autism. Journal of Autism and Developmental Disorders, 35(4), pp. 471-478.
Khashan A, et al, (2008). Higher risk of offspring schizophrenia following antenatal maternal exposure to severe adverse life events. Archives of General Psychiatry; 146-52.
McLafferty M, Ross J, Waterhouse-Bradley B, Armour C (2019). Childhood adversities and psychopathology among military veterans in the US: The mediating role of social networks. Journal of Anxiety Disorders, (65) pp. 47-55.
Rodriguez A, Bohlin G, (2005). Are maternal smoking and stress during pregnancy related to ADHD symptoms in children? Journal of Child Psychology and Psychiatry 46:3, pp 246–254
Stien P, Kendall J (2004). Psychological Trauma and the Developing Brain: Neurologically Based Interventions for Troubled Children. Binghamton, NY: Hawthorne Press.