Please, friends, don't laugh about this topic. I post this because I am interested. I found out that my personality correspondences astonishingly with all the traits assigned to the Sun Sign "Scorpio", the fixed element is water. This is for fun only. Whether you believe in astrology or are interested, I can recommend a book I have in front of me, which I use for this topic. The title is: The Astrology Bible by Judy Hall.
Whoever is interested, the following page of my website tells you everything about my character - well - the stars know ! https://karinbartsch.weebly.com/about-us.html
But there is some research that suggests that our temperaments are innate. At Harvard in 2004, psychologists Kegan and Sidman began longitudinal studies of individual babies and tracked them as they grew older. In one study, they presented babies with unfamiliar stimuli and recorded their reactions. Some babies got upset, crying and thrashing their arms and legs; these babies were highly reactive to their environment. Other babies didn’t get upset and remained calm around new stimuli; the “low reactive” babies.
As the babies grew into children and young adults, their temperaments remained consistent; babies who were “highly reactive” grew up to be more cautious and fearful, while the “low reactive” babies remained sociable and daring even as adults.
It is hypothesized that this is the root of why introverts tire more quickly around lots of people, prefer to have just a few close, deep friendships, and feel re-energized by spending time alone.