Researchers on this level should try to explain what occurs on both of the other two levels. The task of the brain physiologist is reductionist. Perfect knowledge of the brain's role would mean the following: given data on how cognition varies from person to person and from time to time, we can map what brain events underlie both social and life histories. To flesh this out, make the simplifying assumption that the mind performs only four operations when cognizing: classification or CL (of the Similarities sort); liberated logic or LL (of the Raven's sort); practical intelligence or PI (needed to manipulate the concrete world); and vocabulary and information acquisition or VI. And posit that the brain is neatly divided into four sectors active respectively when the mind performs the four mental operations; that is, it is divided into matching CL, LL, PI, and VI sectors.
Through magnetic resonance imaging scans (MRI) of the brain, we get "pictures" of these sectors. Somehow we have MRIs from 1900 that we can compare to MRIs of 2000. When we measure the neurons within the CL and LL sectors, we find that the later brains have "thickened" neurons. The extra thickness exactly predicts the century's enhanced performance on Similarities and Raven's.
As for individual differences, we have pictures of what is going on in the brains of two people in the VI sector as they enjoy the same exposure to new vocabulary. We note that the neurons (and connections between neurons) of one person are better nourished than those of the other because of optimal blood supply (we know just what the optimum is). We note that when the neurons are used to learn new vocabulary, the neurons of one person are sprayed with the optimum amount of dopamine and the neurons of the other are less adequately sprayed. And we can measure the exact amount of extra thickening of grey matter the first person enjoys compared to the second. This allows us to actually predict their different performances on the WISC Vocabulary subtest.
Given the above, brain physiology would have performed its reductionist task. Problem-solving differences between individuals and between generations will both have been reduced to brain functions. It will explain both the tendency of various cognitive skills to be correlated on the individual differences level, and their tendency to show functional autonomy on the societal level. That does not, of course, mean that explaining human cognition on the levels of individual differences or social demands have been abolished. Even if physiology can predict every right and wrong answer of people taking IQ tests, no one will understand why these tests exist without knowing that occupation is dependent on mastering certain cognitive skills (social level) and that parents want to know whether their children have those skills (individual differences).
IQ trends over time turn the pages of a great romance: the cognitive history of the twentieth century. I may have made mistakes in interpreting their significance, but I hope I have convinced you that they are significant. Those who differ about that must, in my opinion, assert one or both of two propositions. That since IQ tests measure g, they cannot possibly signal the ebb and flow of anything else. I doubt anyone will defend that proposition. That nothing save g, or the special factors that fit under the umbrella of g, interests them. I believe that some feel that way, which is sad. They will always view the history of cognition through one window.