Although IQ tests undoubtedly measure a wide range of mental ability and are valuable tools in predicting whether someone is likely to be fitted for a particular job or academic course and are predictors of social outcomes. But identifying useful mental qualities is one thing, assuming that the qualities constitute the totality of intelligence quite another. From early in the history intelligence testing psychologists have attempted to establish that the tests measure a quality of general intelligence which they call “g” – the British psychologist Charles Spearman was the originator of the idea in the early years of the twentieth century. The problem is that there is no absolute proof that “g” exists or that IQ tests measure it. However, what IQ tests do plausibly measure is a general ability which applies to a wide range of mental tasks for there is a strong tendency for individuals to perform similarly across the spectrum of tests which make up an IQ score, for example, a high IQ individual will score strongly on all IQ problems, although not with an exact evenness of performance.
But there is an alternative explanation for why individuals score similarly across the range of IQ tests. This is that what is measured by such tests is a catalogue of different abilities, each dependent on some structural quality of the brain, and that the normal development of the brain is such that each structure from which an ability derives develops in concert with all the other structures and, consequently, the various abilities are kept roughly in step. Put another way, a person with a high IQ scores highly across the range of tests because the brain can only develop in a way which produces structures which are roughly equal in ability.
This is not quite as improbable an idea as it might seem at first glance because normal organic development generally displays just such behaviour, for example, the growth of parts of an organism are normally proportionate to the individual organism’s size. If this alternative explanation is correct, the practical effect of such a brain would be the same as a brain governed by some general principle of intelligence. An analogy would be with computers which have programs designed to tackle the same tasks but which have qualitative differences in power and scope. The case of idiots savants with high level specific abilities, which in any other circumstances but those of the idiot savant would be considered high IQ activities, could be accommodated within the hypothesis that intelligence is a conglomeration of separate abilities rather than being a single entity, for it could be that normal development is arrested in most areas and enhanced in one or two. Indeed, high performing idiots savants provide a serious problem for those who wish to claim that “g” exists and the type of explanation offered by those in the field – that the abilities of such idiots savants are talents rather than intelligence – is scarcely convincing.
Against the idea of discrete abilities forming a suite of intellectual tools which give the impression of a single quality of intelligence is the fact that those with innate deformities and deficiencies of the brain or damage to the brain utilise other parts of the brain to perform functions normally associated with the deficient or damaged parts.
There is the further problem for the idea of ‘g’, namely, that there are clearly some forms of what would be considered high level intellectual activity which do not seem to fall into the obvious realm of IQ tested abilities, for example, literary talent, historical and sociological insight. It is true that those who excel in such fields will probably have a healthy IQ, but it is not obvious how the abilities tested by IQ relate to the abilities displayed in such work. It might be thought that this is evidence for “g” and that performance in subjects such as history and sociology is simply an expression of “g”, that is, “g” is being tested by other means than an IQ test. The problem with that argument is that people with similar IQs, both in overall score and in the shape of the IQ, do not display equal facility at intellectual tasks across the board and the difference in particular abilities cannot be put down to simply differences in upbringing or of temperament.