On The General Factor of Intelligence

The idea of a general factor for intelligence, or g was first hypothesized by Spearman (1904). Spearman noted that people highly skilled in one area seem to also be better at most other things as well. There seemed to be a general factor that influenced all forms of intelligence.

IQ testing is a commonly understood method of intelligence testing, although its validity is constantly being called into question. IQ tests use a score norming procedure that curves until age 16, at which point intellectual maturity is said to be reached, and the further curving of scores ceases. Intelligence, in this case, is defined as whatever causes people to outperform their age group on IQ tests. In general, it has been noted that

Catell (1987) made another contribution to the understanding of a general factor of intelligence by putting forth the idea of a crystallized intelligence represented more education and experience. Fluid intelligence was defined as the ability to reason, form logical relationships, and manipulate abstractions. Catell believed that this fluid intelligence would be related to, if not equated with g. A study by Carroll (1993) concluded that a general factor of intelligence, or a fluid intelligence would account for as much as 50% of the variance of scores within 400 tests.

In a lengthy article by Dennis Garlick, a new model for understanding the general factor of intelligence is put forth.  He uses a connectionist framework to hypothesize that "neural plasticity," or the ability of brains to make major connectional changes in response to experience.  It seems that the brain loses the ability to make these major changes in most areas once it reaches maturity.  Areas responsible for higher abilities like language seem to retain this ability longer.

Garlick puts forth the idea that neural plasticity must be a trait similar to metabolic rate in that everyone has a basically different amount available to them. Since neural plasticity is evolutionarily new, it follows that there would be much more variance in that as compared with an older process like lung construction.  Garlick cites research indicating that the brains of intelligent people process information more efficiently as compared to less intelligent people. Positron Emission Tomography studies show a lower level of glucose use in problem solving tasks when compare to less intelligent controls.

As we age, neural plasticity decreases.  It is most available in childhood, which could explain such phenomena as why it is so much simpler for most children to learn new languages while it can take adults much longer, and be much harder.  Once we reach maturity around age 16, many brain regions lose much plasticity.

It could be that intelligent people are able to either adapt to their environment more efficiently as children, which would explain why children labeled as "bright" possess better reasoning skills than others. It could be also that neural plasticity is maintained for a longer time in the brains of intelligent people.