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The idea that personality is structured hierarchically has been around since the 1940s. In 1947, Eysenck proposed that personality can best be described using four hierarchical levels: specific responses, habits, lower-order traits, and higherorder traits. Specific responses are single acts in specific situations. Habits are specific responses that are similar across situations which are best represented by items in a personality questionnaire. For instance, the item “I smile a lot” refers to the habitual behavior of smiling across multiple situations. Lower-order traits consist of habits that are closely related. For instance, the items “I smile a lot” and “I am full of energy” are closely related and belong to a liveliness lower-order trait. In questionnaires, these lower-order traits are referred to as facets. Finally, higher-order traits (or, in questionnaires, often called trait domains) consist of a number of lower-order traits that are related to each other, but not to other higher-order traits. In Fig. 1, the theoretical and questionnaire trait hierarchies are depicted. One of the most important discussions in personality psychology concerns the number of independent higher-order traits (or trait domains) that provide an optimal description of somebody’s personality. Most dictionaries contain thousands of words referring to behaviors that “distinguish one person from another,” but to use all of these words to provide a description of a person would be far too cumbersome. Thus, both for practical purposes–to provide an easily comprehensible personality description of a person in an assessment–and for research purposes, to efficiently measure personality, personality scholars have …