Biological fitness is a directly observable quantity with well-known causal components, in contrast to the latent 'g' factor of psychometrics. The study of the causes of variation in fitness should therefore be simpler than the study of variation in mental abilities, but a paucity of data has kept the nature of genetic variation in fitness obscure. We can define an 'f' factor as variation creating positive correlations among components of fitness. There is little doubt that such f factor exist. Perturbations of populations such as mutation or environmental change create such patterns of positive correlation. However, natural selection will tend to minimize variation in any f factor, so it is much less clear whether f causes quantitatively substantial genetic variation within populations. Experimental data are consistent with variation in an f factor within some natural populations. As predicted, f is less important in populations where natural selection has had more opportunity to reshape the correlation matrix. Although one can incorporate variation in g into a study of variation in human fitness, the pace of change in our environment suggests that the results would neither reflect the conditions under which g evolved nor predict future evolutionary changes in g.