Levels of introversion/extroversion can be explained by a psychological theory called the Arousal Theory. This theory states that humans have preset levels of arousal in their brains, called the natural level of arousal. However, for some people, their natural level of arousal is too high or too low, which motivates them to seek their optimal level of arousal. Some people’s natural level of arousal is too high, which motivates them to seek calmer, more relaxed activities to lower their brain arousal to an optimal level, whereas some people’s natural level of arousal is low, motivating them to seek out busy, adventurous activities to increase their brain arousal to an optimal level.
How does this relate to introverts and extroverts?
Introverts’ natural level of brain arousal is already high, so they seek out calm activities to lower it to an optimal level. These activities usually involve staying inside and being alone or around few people. Introverts’ optimal level of arousal is LOWER than their natural level of arousal, which is why they stay calm/quiet in an attempt to lower the natural level.
Extorverts’ natural level of brain arousal is low, which is why they seek out adventure and high social interaction to raise it. Extroverts’ optimal level of arousal is HIGHER than their natural level.
In a nutshell, levels of natural brain arousal in people is the explanation for introversion and extroversion.