One of the long standing debates in psychology is the underlying nature of intelligence. What defines intelligence? What behaviours or abilities, as a society, do we deem to be those of an intelligent individual?
Psychologists generally agree that the term intelligence "describes a person's ability to learn and remember information, to recognise concepts and their relations, and to apply the information to their own behaviour in an adaptive way". However, defining the exact nature and inner workings of intelligence is where psychologists are of different opinions. It has even been suggested that intelligence is "that which the intelligence tests measure".
Psychologists generally fall into one of two categories when trying to understand intelligence. Those who believe there is an underlying general' intelligence factor, as proposed by Spearman (1927) and Cattel (1966), and those who believe that there are multiple intelligences operating simultaneously, outlined in the theories of Thurstone (1938) and Gardner(1983).
Factor analysis, a procedure developed by Spearman and Pearson which is "used to determine the minimum number of dimensions or factors that account for the observed relationships (correlations) among subjects responses over a large number of different tests" has further allowed psychologists to understand the nature of intelligence.
Spearman proposed that an individuals intellectual ability was controlled by two factors, the g factor, the measure of general intelligence, and the s factor, which related to the specific ability concerning a particular test. He further defined the g factor as comprising of three principles of cognition': apprehension of experience, the ability to understand what they experience, eduction of relations, the ability to understand relations between two things and eduction of correlates, referring to a subjects ability to apply a rule from one case to another. Spearman believe that a persons performance on any given intelligence test was determined by how well they applied these three principles.
A major strength supporting Spearman's theory can be found in the correlations on tests of differing' intellectual abilities, that is to say how closely the various abilities are related. It's logical to assume that correlations on tests of independent, separate abilities would have correlations approaching zero. But it has been found the correlations amongst various tests of intellectual ability range from 0.3 to 0.7. Or more precisely they have 9 per cent to 49 per cent of their variability in common.
Building on these correlations Spearman employed factor analysis to further attempt to define the nature of intelligence. Essentially the procedure of factor analysis is used to determine which set of tests form groups e.g. If scores from several different people over a number of different tests correlate well with one another then the tests may be measuring the same factor, employing a factor analysis can determine the common element amongst the tests.
Louis Thurstone (1938) employed Spearman's factor analysis over 56 tests and found not one but seven separate factors, or intelligences. Which he named verbal comprehension, verbal fluency, number, spatial visualisation, memory, reasoning and perceptual speed.
In contradiction to Spearman's g factor, Thurstone's results showed a capacity for several intelligences operating independent of each other. With the ability of being able to perform coherently on one factor yet poorly on another. It wasn't until several years later that Eysenck (1957) suggested that a second order factor analysis on Thurstone's results might yield one common factor that could be conceived as a general intelligence.
It was Cattell (1963) that performed this second order analysis, his results showed not one but two factors which he labelled as fluid intelligence, a persons potential or native ability to learn and solve problems and crystallized intelligence, better described as what a person has learnt through the use of their fluid intelligence through exposure to books, schooling, and other various educational experiences. The notion being that if two people were subjected to identical learning opportunities then the individual with the greater fluid intelligence would therefore develop a superior crystallized intelligence. A closer look on factor loadings of tests on fluid and crystallized intelligence support these findings. Tests on figural relations, relationships amongst a variety of figures, and memory for example seem to correlate well with fluid intelligence i.e. a persons native ability but correlate poorly with crystallized intelligence. The opposite is found in tests of crystallized intelligence, e.g. verbal comprehension, where some degree of learning is required by the subject.
Although the idea of a general intelligence is supported well by evidence found through factor analysis it doesn't support other skills that undoubtedly could be deemed as a type of intelligence, e.g. musical abilities.
Gardner (1983) proposed a theory that defined intelligence as falling into seven categories: linguistic intelligence, musical intelligence, logical/mathematical intelligence, spatial intelligence, bodily/kinaesthetic intelligence (the type displayed by athletes) and two types of personal intelligence, intrapersonal, looking into oneself, and interpersonal, an intelligence dealing with feelings and emotions of those around us.
Gardner's theory has the advantage of being based on neuropsychological reality. Evidence can be found with people displaying brain damage, while some abilities are impaired they are able to perform other functions relatively normal. For example, an individual with damage to the frontal lobe can have difficulty accessing social situations, a type of personal intelligence, but can still perform other logical or mathematical tasks.
However multiple intelligences theories are hard to test, Gardner himself provided little evidence as to how these intelligences might be tested. A study on multiple intelligences found substantial g loadings of all purely cognitive tests therefore supporting the idea of an underlying general intelligence and directly contradicting Gardner's theory.
It still remains uncertain the exact nature of intelligence. Both Spearman and Cattell provide convincing evidence for the existence of a g factor through factor analysis. Scores on a variety of tests seem to correlate well with each other showing that perhaps there is an underlying factor governing individuals intellectual performance. However, this position makes it difficult to define non-academic abilities such as musical or social intelligences.
Gardner and Thurstone's proposed theories account for these separate intelligences, backed by neuropsychological evidence, but without an adequate test to measure these abilities separately it leaves them wide open to interpretation, and with further studies might reveal an underlying factor.
Whatever position taken on the nature of intelligence it must be agreed upon that there is certainly much room for debate. Both theories provide convincing evidence but fall short of providing a definitive answer. Whether intelligence is governed by a single underlying trait, or by multiple factors operating independently of each other will only truly be defined with further investigations into the subject.