The fact that psychometric g has many physical correlates proves that g is not just a methodological artifact of the content and formal characteristics of mental tests or of the mathematical properties of factor analysis, but is a biological phenomenon. The correlations of g with physical variables can be functional (causal), or genetically pleiotropic (two or more different phenotypic effects attributable to the same gene), or genetically correlated through cross-assortative mating on both traits, or the nongenetic result of both being affected by some environmental factor (e.g., nutrition). The physical characteristics correlated with g that are empirically best established are stature, head size, brain size, frequency of alpha brain waves, latency and amplitude of evoked brain potentials, rate of brain glucose metabolism, and general health.
The general factor of learning and problem-solving tasks in infrahuman animals has some properties similar to the g factor in humans, and experimental brain lesion studies suggest that a task's loading on the general factor is directly related to task complexity and to the number of neural processes involved in task performance.
It is clear that g, since it is a product of human evolution, is strongly enmeshed with many other organismic variables.
Hierarchical factor analysis has solved the taxonomic problem of dealing with the myriad mental abilities that have been observed and measured. The factors discussed so far, however, concern variables entirely within the realm of conscious, intentional performance on psychometric tests, wherein g appears as a predominant and ubiquitous factor.
Is g a phenomenon that is entirely confined to the psychometric and behav-