"A lot of what extroversion is about is being more motivated by the possibility of reward. ... One of the ways that I like to think about dopamine: It has been described as essentially regulating the cost of effort. Doing stuff has a cost. ... Extroverts just happen to be tuned in such a way that they often think it's a good idea to expand energy. Dopamine basically gives you the signal: 'Something good could happen if you put some effort out right now.' "