Reciprocal causation between mind and brain entails that our brains may well be different from those of our ancestors. It is a matter of use and structure. If people switch from swimming to weight-lifting, the new exercise develops different muscles and the enhanced muscles make them better at the new activity. Everything we know about the brain suggests that it is similar to our muscles. Maguire et al. (2000) found that the brains of the best and most experienced London taxi-drivers had an enlarged hippocampus, which is the brain area used for navigating three-dimensional space. Here we see one area of the brain being developed without comparable development of other areas in response to a specialized cognitive activity. It may well be that when we do "Raven's-type" problems certain centers of our brain are active that used to get little exercise; or it may be that we increase the efficiency of synaptic connections throughout the brain. If we could scan the brains of people in 1900, who can say what differences we would see?
Do huge IQ gains mean we are more intelligent than our ancestors? If the question is "Do we have better brain potential at conception, or were our ancestors too stupid to deal with the concrete world of everyday life," the answer is no. If the question is "Do we live in a time that poses a wider range of cognitive problems than our ancestors encountered, and have we developed new cognitive skills and the kind of brain that can deal with these problems?," the answer is yes. Once we understand what has happened, we can communicate with one another even if some prefer the label "more intelligent" and others prefer "different." To care passionately about which label we use is to surrender to the tyranny of words. I suspect most readers ask the second question, and if so, they can say we are "smarter" than our ancestors. But it would probably be better to say that we are more modern, which is hardly surprising!
The theory of intelligence
The thesis about psychometics and cognitive history, that they actually complement one another, and the remarks made about the brain imply a new approach to the theory of intelligence. I believe we need a BIDS approach: one that treats the brain (B), individual differences (ID), and social trends (S) as three distinct levels, each having equal integrity. The three are interrelated and each has the right to propose hypotheses about what ought to happen on another level. It is our job to investigate them independently and then integrate what they tell us into a coherent whole.
The core of a BIDS approach is that each level has its own organizing concept and it is a mistake to impose the architectonic concept of one level on another. We have to realize that intelligence can act like a highly correlated set of abilities on one level (individual differences), like a set of functionally independent abilities on another level (cognitive trends over time), and like a mix on a third level (the brain), whose structure and operations underlie what people do on both of the other two levels. Let us look at the levels and their organizing concepts.
Individual differences: Performance differences between individuals on a wide variety of cognitive tasks are correlated primarily in terms of the cognitive complexity of the task (fluid g)—or the posited cognitive complexity of the path toward mastery (crystallized g). Information may not seem to differentiate between individuals for intelligence but if two people have the same opportunity, the better mind is likely to accumulate a wider range of information. I will call the appropriate organizing concept "General Intelligence" or g, without intending to foreclose improved measures that go beyond the limitations of "academic" intelligence (Heckman & Rubenstein, 2001; Heckman, Stixrud, & Urzua, 2006; Sternberg, 1988, 2006; Sternberg et al., 2000).
Society: Various real-world cognitive skills show different trends over time as a result of shifting social priorities. I will call this concept "Social Adaptation." As I have argued, the major confusion thus far has been as follows: either to insist on using the organizing concept of the individual differences level to assess cognitive evolution, and call IQ gains hollow if they are not g gains; or to insist on using the organizing concept of the societal level to discount the measurement of individual differences in intelligence (e.g. to deny that some individuals really do need better minds and brains to deal with the dominant cognitive demands of their time).
- The brain: Localized neural clusters are developed differently as a result of specialized cognitive exercise. There are also important factors that affect all neural clusters such as blood supply, dopamine as a substance that render neurons receptive to registering experience, and the input of the stress-response system. Let us call its organizing concept "Neural Federalism." The brain is a system in which a certain degree of autonomy is limited by an overall organizational structure.