How do we know that there are two fundamentally different types of general intelligence. We know this first because the subtests that we describe are as performance-oriented clearly draw relatively more on reasoning skills (fluid intelligence) than on knowledge (crystallized intelligence), and the subtests that we call verbal clearly depend relatively more on knowledge (including knowledge about algorithmic solutions) than on reasoning skills. Also, the verbal subtests have higher correlations with one another than they do with the performance subtests, and vice versa. ~ Page 9

In addition, the subtests ahat we call measures on fluid intelligence rest on executive functions that are mediated by portion of the frontal cortex called the prefrontal cortex (PFC) and another region linked in a network with the PFC called the anterior cingulated. The destruction of the PFC has devastating consequences for mental tasks that require the executive functions of working memory, attentional control, and inhibitory control. People with severe damage to PFC may be so incapable of solving Raven matrices portal.wpspublish.com/portal/page?_pageid=53,69636&_d... that they function at the level of mentally retarded people on that test, yet have crystallized intelligence that is entirely normal. The opposite pattern also occurs. Autistic children usually have impaired crystallized intelligence but may have normal or even superior fluid intelligence.

As one would expect given the lesion evidence, the PFC is particularly active, as demonstrated in brain imaging studies, when people attempt to solve problems that make substantial use of fluid intelligence, such as the Raven matrices and difficult mathematical problems.

Additional evidence for the two types of intelligence is that fluid intelligence and crystallized intelligence have quite different trajectories over a life time. Figure (above) shows an idealized version of those trajectories. The growth of fluid intelligence is rapid over the first years of life but begins to decline quite early. Already by the early twenties, fluid intelligence shows some decline. Mathematicians and others who work with symbolic, abstract materials for which they must invent novel solution may find their powers fading somewhat by the age of thirty. By seventy years old, fluid intelligence is noticeably less than it has been – more than one standard deviation lower.

Older people find it harder to solve puzzles and mazes. Crystallized intelligence, on the other hand, may continue to increase over the lifespan, at least until very old age. Historians and others whose best work depends on a large storehouse of information may find their powers increasing into their fifties.

Note that everything I have just written about the age trajectories and fluid and crystallized intelligence is controversial to some degree. I will spre you the ins and outs of this controversy and simply say that the universally agreed-upon fact is that fluid intelligence begins to decline earlier than crystallized intelligence.

The fluid intelligence declines earlier in life than crystallized intelligence could be predicted by the fact that the PFC shows deterioration earlier than other structures in the brain.

A final source of evidence that the two types of intelligence exist is that executive functions and overall IQ may be separately heritable. Executive functions are inherited to a degree from parents, and so is crystallized intelligence, or the knowledge that helps people solve problems. A person can inherit relatively high executive function from parents who have relatively high executive function, yet can inherit relatively low crystallized intelligence from the same parents who score relatively low on this dimension.

Fluid intelligence is more important to good intellectual functioning for younger people than it is for older people. For young children, the correlation between fluid intelligence and reading and math skills is higher than that for crystallized intelligence. In contrast, for older children and adults, the correlation between crystallized intelligence and reading and math skills is higher than that for fluid intelligence. ………. Pages 10 & 11 )Intelligence and How to Get it by Richard E. Nisbett