Nor are introverts necessarily shy. Shyness is the fear of social disapproval or humiliation, while introversion is a preference for environments that are not overstimulating. Shyness is inherently painful; introversion is not. One reason that people confuse the two concepts is that they sometimes overlap (though psychologists debate to what degree). Some psychologists map the two tendencies on vertical and horizontal axes, with the introvert-extrovert spectrum on the horizontal axis, and the anxious-stable spectrum on the vertical. With this model, you end up with four quadrants of personality types: calm extroverts, anxious (or impulsive) extroverts, calm introverts, and anxious introverts. In other words, you can be a shy extrovert, like Barbra Streisand, who has a larger-than-life personality and paralyzing stage fright; or a non-shy introvert, like Bill Gates, who by all accounts keeps to himself but is unfazed by the opinions of others.