ABSTRACT
At one level, it is difficult to understand what all the fuss is about. g can be thought of as simply a label for the empirical phenomenon of “positive manifold.”1 This is the ubiquitous finding of positive correlations among scores on tests of cognitive ability, first noted by Spearman (1904). Take any set of cognitive tests, and the correlations among them will all be positive, unless there is something very odd about the sample of test takers (e.g., sample too small, test takers prescreened to be identical in ability level) or the tests (e.g., unreliable, much too hard or too easy for the group of test takers).
