Most modern theories of personality are structured hierarchically, with broad higher-order dimensions predicting narrower trait-level variables. This approach is necessarily reductionist, proposing to summarize the majority of trait-specific variability with a smaller number of larger dimensions. Not surprisingly, therefore, significant research has been directed to determining what might be considered to be the most basic dimensions of personality—both with regards to the identification of how many factors are needed to describe personality, and what these factors might be called (Costa & McCrae 1992a; [29] and [30]; Tupes & Christal 1992; Zuckerman 1992; [121] and [122]. While these omnibus factors of personality have proven useful in the prediction of broadly defined behavioral criteria (Paunonen 2003), they raise the important question of “what makes a factor basic?” (Zuckerman 1992). Zuckerman (1992) suggested four characteristics that are critical to the identification of a “basic trait”: (1) replication across methods, genders, ages, and cultures; (2) at least moderate heritability; (3) evidence of similar “personality” traits in non-human species; and (4) at least a partial foundation in biology.