"The Big 5" model of personality theorizes that personalities can be described using a sliding scale for each distinct trait. For example, on one side of the openness scale, you'd find someone who prefers routine, while the other side would indicate a preference for variety and independence. These five traits are fairly universal -- they don't change depending on a person's culture, and are largely considered to be biological. (You can remember the model with the anagram OCEAN or CANOE.) That said, some researchers believe there are many more than five personality traits. The late psychologist Gordon Allport proposed that there were 4,000 distinct traits that made up a person's personality.