Are you an extrovert or an introvert? If you've ever done a Myers-Briggs test, you are familiar with the questions: Do you prefer being in a crowd when you are stressed, or would you rather retreat? Do you have lots of friends, or just a few, very deep ones? Do you prefer a loud party of 100 or an intimate gathering of 4?
While in parts of the business world the Myers-Briggs Type Indicator is almost a religion, it was originally meant as a tool to explain that different people think differently, and that these differences of manner shouldn't be read as differences of attitude. It's not that a Perceiver is indecisive - it's that a P needs data to form an opinion. It's not that a J is impatient, it's that until the thing is finished, it may well not exist to her.
The I vs. E dimension, introversion vs. extroversion, is something many people easily identify with, much more so than the other three. In fact, while all other ones require explanation just for anyone to understand what they mean, with this dimension it's the other way around: people identify first, and then they usually need to be explained what the dimensions mean.
The problem with this dimension is that many people that aren't on the ends of the spectrum don't quite know what to make of the classification. They feel neither introverted nor extroverted, but they don't particularly identify with other "zeros". What's going on?
Primary human emotions are few and powerful. Research tells us there are six of them: happiness, sadness, fear, anger, surprise and disgust. They are very distinct and come from different places in our brain, having evolved out of reactions to external influencers. Fear, for instance, is an emotion meant to deal with threats. Disgust is supposed to prevent us from doing things (usually: eating them) that could be dangerous for us. Surprise is the way we deal with the unexpected and opens us up for new information.