Researchers at Northwestern conducted a study of over 1.5 million people and identified four major "clusters" that people fit into. "Average" is the largest group, with people who tend to be neurotic and extroverted, but not very open. "Reserved" people are more emotionally stable, but tend to be more introverted. "Role Models" are dependable, less neurotic, and make better leaders. As people age, more of them fit into this category. Finally, "Self-Centered" people are highly extroverted but lower in most other categories. Teenagers are frequently in this category and many people eventually age out of it.