How often do you describe a friend as an introvert, to excuse his shyness, or an extrovert, to explain her fondness for karaoke? In fact, these definitions only came into common use at the end of the second world war.
Psychologist Hans Eysenck was working with traumatised soldiers when he began to create classifications for different personality types.
One of the key differences Eysenck found was how much comfort a person took in the company of others - whether they thrived in the presence of other people, or whether they seemed to crave time on their own, or in a much smaller group. Although other psychologists had identified this before, Eysenck's work differed because he believed we all sat somewhere on a continuum of behaviour, rather than in different camps. Eysenck's research popularised the terms "introvert" and "extrovert" and changed the way that most of us think about ourselves and others.
Now, all kinds of traits are associated with introversion and extroversion. Studies show, for example, that extroverts may have a higher sensitivity to dopamine and that they prefer to wear more decorative clothing. Although studies of twins indicate that some of our introvert/extrovert traits may be genetic, most psychologists believe it is possible for us to overcome our natural state if a situation requires us to act differently.