Research shows that some personality traits like introversion and extraversion have biological or physiological origins. Eysenck (1983) spearheaded much of the research on the physiology of introversion/extraversion, showing that introverts are essentially more aroused than extraverts. Gale (1983) and Stelmach (1990) have pointed out some methodological weaknesses in Eysenck’s work but generally the principle that extraverts can handle stimulation or distraction better than introverts has seemed to correspond to the differences in these temperaments. The results of these studies have tremendous implications for everything from classroom design to communications. For example, O’Connor, Gardiner & Watson (2016) found that introverts benefitted more from relaxation techniques to stimulate creativity, whereas extraverts benefitted more from ideational skills training. As someone who is basically an introvert with some extraverted tendencies, I can see why there are some inconsistencies in the research. A pure introvert or extravert might react as expected on physiological tests like the eye blink or salivation test, but different people will react differently to different situations depending on a number of different variables like conditioning, habituation, and conditioning. In fact, some neurological studies even seem to reveal the opposite results of the Eysenck studies. For instance, Suslow,...

Kou, McClelland & Furnham (2017) found no differences between extraverts and introverts in terms of performance on complex tasks with or without background noises. The results of this and similar studies show that culture, context, and individual differences may be more important for predicting arousal, anxiety, and other states than introversion/extraversion.
Self-concept is also relevant to extraversion/introversion. I had been told that I was shy and introverted. Some cultures reward introversion, and others reward extraversion. The role of conditioning and social norms needs to be taken into consideration when evaluating subjects for introversion or extraversion. At the same time, the physiological studies also add credence to the otherwise soft personality tests used by human resources departments. If personality traits like introversion or extraversion are rooted in biology, then it would also make sense why some people react to drugs and alcohol differently than their peers. The problem is that reactions to drugs and alcohol is also based on other variables, like the setting in which the substances were taken, the person’s pre-existing emotional state, diet, and other factors.

Introverts are usually considered shy, withdrawn, and either anxious…