Spearman's g: links between psychometrics and biology.

Individual differences with respect to diverse tests of mental abilities that range in complexity from simple reaction time to abstract reasoning are all positively correlated in the population. The total covariance among all such tests can be analyzed into a number of uncorrelated components of variance, or factors, that, in terms of their generality, are hierarchical, with the most general factor, or g, at the apex. This g factor is common to every type of cognitive performance, whatever other ability factors may be involved (e.g., verbal, spatial, numerical, musical, etc.), and is the crucial factor in most tests' practical validity. Its correlations with various tests' heritability, inbreeding depression, heterosis, average evoked potentials, brain metabolism, and many other physical correlates indicate that as a product of evolution it is profoundly enmeshed with many organismic variables. A theory based on empirical evidence links g to neural processes involved in the speed and efficiency of information processing.