 If Big Five traits are not real entities with causal powers, then what are they? The standard view is that they are just descriptive labels for how people tend to be. Put differently, they are useful devices—terms of classification—which are far more economical to use than is appealing to the thousands of trait terms in the English language. Grouping people into only five categories is thus highly efficient (Hogan 1996: 170–173; McCrae and Costa 2003: 36), but it is a further step to say that there actually is a trait of extraversion which causally explains individual differences between people. As Daniel Ozer and Steven Reise note, the Big Five taxonomy, 