There are many psychologically relevant reasons for preferring factor solutions that contain a "g" factor. These include the existence of the positive manifold, the fact that certain kinds of tests (generally the more complex ones) have consistently larger "g" loadings, the substantial invariance of "g" factors across different test batteries, the impossibility of constructing test batteries that do not yield a "g" factor, and the widespread practical validity of "g" as a predictor of individual outcomes. The "g" factor, together with group factors, best represents the empirically established fact that, on average, overall ability differences "between" individuals are greater than differences among abilities "within" individuals, while a factor solution with orthogonal factors without "g" obscures this fact. Moreover, "g" appears to be the most heritable component of intelligence. Research utilizing the techniques of confirmatory factor analysis has also provided support for the existence of "g".